One of the most controversial issues in biology since the time of Locke and Leibniz has been that of the role ex- erted by heredity and environment- what is inborn and what is acquired. Two factions arose in attempting to explain this delicate balance. One mini- mized the role of genetics as a deter- minant of behavior, and the other (mainly represented by ethologists and geneticists) emphasized the importance of heredity (1). As a matter of fact, the genetic de- terminants of behavior have been con- fused with, or identified with, fixed ac- tion patterns currently classified as instinctive. Heredity, however, also affects many forms of behavior less organized or less systematic than nest building, sexual behavior, or some forms of highly organized aggressive- ness. Each individual or species is pro- vided with genetic baggage responsible for the so-called spontaneous motor activity, for the search for new stimuli (which are the grounds of the explora- tory behavior and curiosity), and, more generally, for all types of motivational and emotional behavior. Besides, any type of adaptive behavior, like the ap- titude to respond to a conditioned stim- ulus, to learn, and to remember, is genetically determined. Heredity plays an important role in different forms of adaptive behavior, since individual re- actions to environmental influences and the ability of man to benefit from cul- tural heritages depend on hereditary factors. Due to the difficulties of controlling the genetic and environmental determi- nants and the role played by cultural factors in human societies, it seems necessary to start with experimentation on animals in order to confront the 10 JANUARY 1969 problem of "nature and nurture" in men. The use of other animal species involves a less-complex approach. The problem of nature and nurture consti- tutes the target of interest in today's psychobiology, a new branch of biology that affords a tentative bridge between neurophysiology and comparative psy- chology, and ethology. Psychobiology attempts to study whether, and to what extent, learning behavior is ge- netically determined. Behavioral Genetics One of the first extensive experi- ments in the field of psychogenetics was Tryon's selective breeding program (2). In mating rats which displayed characteristic traits, Tryon selected those among their offspring with the highest or lowest performance in a standardized maze and bred their off- spring. Bright maze learners produced bright progeny, whereas dull maze learners produced dull progeny. Tryon's experiments, based on carefully con- trolled breeding and environmental factors, established the inherited na- ture of this adaptive behavioral trait. In light of these experiments it seemed advisable to use animals bred selec- tively, from genetically homogeneous strains to assess the effects of heredity on behavioral aptitudes (3). In agreement with Hall (4), the main objectives of psychogenetics are (i) to discover whether a given be- havior pattern is transmitted from gen- eration to generation, (ii) to determine the number and nature of the genetic factors (5) involved in the trait, and (iii) to locate the genes on the chromo- somes. We discuss here the first of the above problems. Genetic Aspects of Learning and Memory in Mice The study of differences among strains and individual subjects is a most promising topic in psychobiology. Daniel Bovet, Filomena Bovet-Nitti, Alberto Oliverio after the onset of the light. A series of Dr. Bovet is professor of pharmacology and chairman of the Department of Pharmacology, University of Sassari, Italy, and is visiting pro- fessor of physiology at the University of California, Los Angeles. Dr. Bovet-Nitti and Dr. Oliverio work closely with Dr. Bovet in his studies. 139 Scott, Frederickson, and Thompson (6) demonstrated that inbred strains of mice differ not only with respect to certain morphological traits but also with regard to behavioral traits. As far as adaptive behavior is concerned, the acquired behavior of inbred mice has been studied by the use of a reward as motivation; in a few investigations, avoidance conditioning has been used (7). Individual and Strain Performances in Avoidance Behavior Our experiments with different con- ditioning tests for screening the action of psychotropic agents demonstrated that the large individual variability of a heterogeneous population was an obstacle to the assessment of the factors affecting the patterns of learning and retention, characteristic of a given spe- cies. By the use of different strains of inbred mice, we developed an avoidance technique similar to one which we had used for the rat. In this paper we de- scribe results obtained by studying the avoidance behavior of different strains of mice in a shuttle box. Double-Compartment Grill-Box Technique The apparatus used for the study (8) of escape and avoidance responses is a two-compartment cage adapted from Warner's model (9). Each apparatus consists of a rectangular plexiglass box divided into two equal compartments which are connected by a small open- ing (Fig. 1). The floor is a tilting plat- form of stainless steel rods. On each trial the conditioned stimulus (a lamp which lights the compartment where the animal is) is preceded by a constant interval (5 seconds), and the uncon- ditioned stimulus is represented by a continuous electric shock administered through the grid floor. An uncondi- tioned escape response is recorded when the mouse shuttles into the adja- cent compartment after the onset of the shock. A conditioned avoidance response is recorded when the mouse avoids the shock by running into the other compartment within 5 seconds