In order to examine the role of hypothalamic SP in the feedback regulation of prolactin, we studied the effect of prolactin and dopamine on SP concentration and release, and the effect of SP on dopamine release. Hypothalamic fragments from male Wistar rats were incubated in the presence of prolactin, dopamine or SP under basal and K(+)-stimulated conditions. SP (10(-7) M) stimulated dopamine release, while dopamine (10(-7) M) decreased SP content and release. Prolactin (100 ng ml-1) increased SP content and release. An increase in hypothalamic SP content was also found during suckling. In addition, a specific antagonist for SP, Win 62,577, blocked the effect of prolactin and dopamine release. These results show an interaction between SP and dopamine at the hypothalamic level and suggest that SP could mediate the feedback action of prolactin on dopamine release.