In the male rat, hyperprolactinaemia is associated with significant reductions in plasma LH and FSH levels and in several measures of copulatory behaviour. In contrast to this situation, experimental induction of hyperprolactinaemia in male mice and hamsters is associated with an increase in plasma gonadotrophin levels. It was therefore of interest to determine the effects of hyperprolactinaemia on the copulatory behaviour of these animals. Hyperprolactinaemia was induced by transplantation of pituitaries from adult females and sexual behaviour was tested in the presence of ovariectomized, oestrogen- and progesterone-treated females. Because hyperprolactinaemia increases plasma testosterone levels in intact male hamsters, the animals were castrated and implanted with testosterone-filled silicone elastomer capsules before induction of hyperprolactinaemia. In mice of two inbred strains, DBA/2J and C57BL/6Bg, hyperprolactinaemia appeared to stimulate male sexual behaviour as shown by a significant increase in the proportion of animals mating (C57BL/6) and a significant decrease in mount (DBA/2J) and intromission (C57BL/6Bg and DBA/2J) latencies. Similarly, hyperprolactinaemia did not suppress male copulatory behaviour in the hamster. In contrast, in two experiments in which the animals were tested three times for sexual behaviour, mount or intromission latencies were significantly reduced in pituitary-grafted, as compared with sham-operated males, in the first of the tests. Thus, in the mouse and the golden hamster, experimentally induced chronic hyperprolactinaemia stimulates both gonadotrophin release and male copulatory behaviour. These observations, together with the association of suppressive effects of hyperprolactinaemia on plasma LH and FSH levels and on sexual behaviour in the male rat, suggest the possible existence of a common mechanism underlying both endocrine and behavioural effects of hyperprolactinaemia.(ABSTRACT TRUNCATED AT 250 WORDS)