Hyperprolactinaemia is an important cause of infertility due to the suppression of gonadotrophin release. In seasonal breeders such as the sheep and horse, PRL displays a temporal pattern of secretion. Peak secretion of prolactin (PRL) coincides with the long days of summer in both species, yet whereas the sheep is infertile during the sununer, the horse is fertile. Moreover, in both species, PRL only suppresses the gonadotrophin response to GnRH when applied to pituitary cultures concomitantly with bromocriptine (Br), a dopamine (DA) agonist, during the summer. Therefore, the suppressive effects of PRL and Br are dependent on photoperiod/season and independent of reproductive status. The aim of the current studies was: 1) to characterise the interactions between the GnRH, dopamine and PRL axes at the level of the gonadotroph; and 2) to elucidate the intrapituitary mechanisms which seasonally regulate the suppressive effects of PRL and Br on the gonadotrophin responses to GnRH.