Prior exposure of the rat ventral prostate to a high dose of ovine prolactin in vivo or in vitro caused the specific desensitization of the prostaglandin E1-dependent adenylyl cyclase. Maximal desensitization was achieved in vivo 6 h after exposure to 100 micrograms prolactin and complete resensitization occurred after 7 days. No heterologous desensitization of the response of the cyclase to other hormones tested was observed. Incubation of isolated secretory cells with prolactin and prostaglandin E1 desensitized the subsequent response to the same prostaglandin. Simultaneous exposure of the cells to indomethacin prevented the prolactin-induced effect. Prolactin could also induce similar changes in the prostaglandin E1-dependent cyclase of rat Leydig cell tumour cells in vivo, suggesting that this may represent a phenomenon shared by other prolactin target organs. This study supports the hypothesis that prostaglandins may constitute second messengers for prolactin effects on the prostate gland.