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The relationship between the testes and the adenohypophysis in rats exposed to heat was assessed by (1) the determination of adenohypophysial concentrations of FSH, LH and prolactin as well as serum levels of prolactin and testosterone in rats maintained at 35 °C for various periods of time and (2) the elucidation of the responsiveness to stimulation with gonadotrophin releasing hormone (GnRH) or human chorionic gonadotrophin (hCG) of the rat acclimatized for 40 days to a temperature of 35 °C. The results demonstrated that while heat exposure caused a transient change in the concentration of FSH and LH in the adenohypophysis, the level of prolactin increased by twofold within 24 h and remained significantly higher than in control animals. A concurrent significant decrease in the concentrations of prolactin and testosterone in the serum of rats exposed to heat was also observed. Challenging the rat with exogenous GnRH resulted in a significant increase in serum LH and FSH concentrations in control rats and rats exposed to heat, but the magnitude of the rise in LH was greater in the latter group. However, this phenomenon was not accompanied by a rise in serum testosterone concentration in the rat exposed to heat. Challenging the animals with hCG provoked an increase in serum testosterone, but the magnitude of the rise was greater by far in the control rats. It is concluded that exposure of the intact rat to increased environmental temperatures is accompanied by accumulation of prolactin in the lactotrophs, reduced serum prolactin concentration, decreased capacity of the testes to synthesize testosterone and consequently reduced serum testosterone concentration. These phenomena are independent of the capacity of the adenohypophysis to synthesize LH.