298 Experimental Oncology 27, 298-302, 2005 (December) Breast cancer is the third most common malignant neoplasm worldwide and in India it is the second most common cancer after cervix accounting for 19% of the all cancer burden in women [1]. The incidence of breast cancer is influenced by familial, endocrine and environmental factors [1, 2]. Imbalance in oxidants and antioxidants is known to be involved in the pathogenesis of various diseases including breast cancer. It has been reported that lipid peroxidation by-products have a crucial role in the early phases of tumor growth if they are exces- sively generated [3]. The ability of lipid peroxidation by-products in generating mutagenesity and DNA adducts formation suggest their possible role in car- cinogenesis [4]. The studies have been published on the status of free radicals and oxidative defense mechanism in different types of malignancies [5–7]. Previous studies have however reported controversial and conflicting results with regard to the levels of lipid peroxidation by-products and antioxidants in human breast cancer [8, 9]. Lower plasma malondialdehyde (MDA) levels [8] and higher plasma MDA levels [9] have been reported in patients with breast cancer. Thus measurement of daily changes of oxidants and antioxidants may provide solution for the conflicting results on the relations of blood oxidant and antioxidant status in mammary carcinogenesis. Biological clock controlling the endogenous cir- cadian rhythms of most aspects of physiological and biochemical processes in mammals is known to reside in suprachiasmatic nucleus of the hypothalamus and the primary external signal that entrains the biologi- cal clock is light-dark cycle [10, 11]. Circadian system alterations have been described in tumor tissue, tumor bearing animals and cancer patients [12]. It has been documented that fast growing poorly differentiated tumors showed less pronounced rhythms than slow growing well-differentiated tumors [13]. Oxidative and enzymatic antioxidative indices were shown to posses circadian variations in experimental animals [14, 15]. Studies on diurnal rhythm of plasma TBARS and antioxidants can provide the solution for previous conflicting and controversial reports on the status of TBARS and antioxidants in mammary car- cinogenesis. The ultimate goal of the present study is therefore to provide a clear picture on temporal pattern of TBARS and antioxidants in experimental mammary carcinogenesis. MATERIALS AND METHODS Female Wistar-albino rats, 7–8 weeks old, weighing 130–140 g were used for the study. The animals were obtained from central animal house, Rajah Muthiah Institute of Health Science, Annamalai University, Annamalai Nagar, India. The rats were housed in polypropylene cages at room temperatures (30 ± 2 °C) under semi natural conditions in experimental room. In Annamalai Nagar, the LD (Light : Dark) cycle is almost 12 : 12 h. All animal procedures were approved by Local Ethic Committee. A total number of 20 animals were divided into 2 groups of 10 each, and 4 rats in each group were used separately for histopathological studies at the end of the experimental period. Mammary carcinoma were induced in group-I rats by subcutaneous injection of single dose of 25 mg 7,12-dimethylbenz(a)anthrace- Exp Oncol 2005 27, 4, 298-302 DIURNAL RHYTHMICITY OF THIOBARBITURIC ACID REACTIVE SUBSTANCES AND ANTIOXIDANTS IN EXPERIMENTAL MAMMARY CARCINOGENESIS K. Kolanjiappan, S. Manoharan* Department of Biochemistry, Faculty of Science, Annamalai University, Annamalainagar 608002, Tamil Nadu, India Aim: Circadian rhythms of reactive oxygen species and antioxidants have been the subject of considerable interest in recent years due to their crucial role in the development of malignant tumors. Our aim was to analyze the circadian pattern of lipid peroxidation by-products (TBARS) and antioxidants status in 7,12-dimethylbenz(a)anthracene (DMBA) induced rat mammary carcinogenesis. Methods: Mammary tumors were developed in Wistar rats, blood samples were collected at a regular interval of 4 h throughout the 24 h period and TBARS content and antioxidant enzymes activities (superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx)) were estimated by colorimetric methods. The Cosinorwin computer software program was used to analyze the characteristics of biochemical rhythms. Results: In mammary carcinogenesis, the acrophase of thiobarbituric acid reactive substances and antioxidant enzymes (SOD, CAT, GPx) in blood were found to be delayed, whilst mesor values for TBARS level — increased, and values for SOD, CAT, GPx activities — decreased. Significant r-values indicate the disruption of lipid peroxidation-antioxi- dants rhythms in mammary carcinogenesis. Conclusion: The present investigation suggests that the circadian pattern of blood lipid peroxidation and antioxidants in rats with mammary carcinoma are markedly desynchronized as compared to normal rats. Key Words: mammary carcinogenesis, DMBA, lipid peroxidation, antioxidant enzymes. Received: July 26, 2005. *Correspondence: Fax: 0091-4144-238145 E-mail: manshisak@yahoo.com Abbreviations used: CAT — catalase; GPx — glutathione peroxidase; SOD — superoxide dismutase; TBARS — thiobarbituric acid reactive substances.