Abstract: Various clinical observations and experimental data from in vitro studies suggest that insulin and sex hormones interact. The main purpose of the present study was to examine the effects of testosterone and estradiol on serum insulin in rats. Seven week old male and female albino (Wistar) rats were used in our study. Testosterone enanthate (50 mg kg-1 day-1) or estradiol valerate (200 μ g kg-1 day-1) were injected intraperiotenally or subcutaneously in orchidectomised or ovariectomised rats, respectively. In orchidectomised rats, serum insulin was decreased compared with control animals (p<0.01), on the other hand, decreasing of serum insulin was prevented by testosterone replacement (p<0.001). In ovariectomised rats, serum insulin was also decreased compared with control group (p<0.01) and decreasing of serum insulin was prevented by estradiol replacement (p<0.05). Conclusively, present findings indicated that testosterone or estradiol were serum insulin enhancer hormones in male or female rats, respectively.