J. Exp. Biol. (1967), 46, 329-337 329 With 5 text-figures Printed in Great Britain CHANGES OF GAS CONCENTRATIONS IN BLOOD AND WATER DURING MODERATE SWIMMING ACTIVITY IN RAINBOW TROUT BY E. DON STEVENS AND D. J. RANDALL Zoology Department, University of British Columbia, Vancouver, B.C., Canada (Received 4 November 1966) INTRODUCTION In a previous paper (Stevens & Randall, 1967) changes in blood pressure, heart rate and breathing rate during activity were reported. Interpretation of these results as to the physiological significance of such changes can be expanded with a knowledge of any concurrent changes in gas concentrations in blood and water afferent and efferent to the gills. Recent technical developments permitting gas analysis on small quantities of blood, associated with the development of suitable handling and cannulation tech- niques (Holeton & Randall, 1967 a, b), allow measurement of blood-gas concentrations in unanaesthetized, unrestrained, intact fish. The object of this study was to measure changes in gas concentrations in blood and water afferent and efferent to the gills of rainbow trout before, during, and after moderate swimming activity. METHODS The experiments were carried out on fifty hatchery-raised rainbow trout (Salmo gairdneri) weighing between 200 and 400 g. Thefishwere maintained and cannulated as previously described by Stevens & Randall (1967). When all cannulae were in place, a fish was placed in a respirometer tube similar to that described by Brett (1964). After recording pre-exercise levels, the water velocity was increased stepwise once every minute for 5 min., maintained at a maximum level for 5 min. (51-8 cm./sec), and then decreased stepwise once every minute for 5 min. Blood PQ 2 and blood P COi] were determined separately on small blood samples with electrodes in a Beckman Modular Cuvette and a Beckman Physiological Gas Analyser (model 160). P Oa was measured with a Beckman oxygen macro-electrode and P CO2 was measured with a Severinghaus-type P COa electrode. The blood sample was maintained at the same temperature as the fish during the determination, and was returned to the fish after each determination. In any single experiment blood was sampled from only one blood vessel and only P Oj or Pco 2 was determined. Blood-gas concentrations could not be determined every minute because of the slow response time (1-5 min.) of the electrodes as low temperatures. Oxygen consumption was measured on eleven fish in the respirometer by periodi- cally permitting the fish to utilize about 10% of the oxygen available (Brett, 1964). Oxygen was determined using the unmodified Winkler method.