Prolactin (1, 5, 10 micrograms/ml) inhibited 5 alpha-dihydrotestosterone (DHT), testosterone (T) and estradiol-17 beta (E2) secretion by luteal cells isolated from porcine corpus luteum on the 13th day of the estrous cycle. The effect of prolactin upon luteal cells on the 5th and 17th day of the cycle was different. Inhibition of DHT, T and E2 secretion in luteal cells on the 5th day of the cycle occurred only after 6 hours of incubation, at prolactin doses of 5 and 10 micrograms/ml of the medium. Luteal cells obtained from corpora lutea with considerable physiological luteolysis (17th day of the cycle) did not react to the presence of prolactin in the incubation medium. The results showed that the effect of prolactin upon luteal cells depended on the intensity of steroidogenesis. Luteal cells on the 5th day of the estrous cycle produced progesterone in the amounts 34 +/- 9.1 ng/ml, on the 13th day: 62 +/- 11.5, and on the 17th day: 7.9 +/- 1.9. The effect of prolactin was most pronounced in case of cells on the 13th day of the cycle. These cells were characterized by the highest production of progesterone. The in vitro observations suggest the role of prolactin in inhibiting synthesis or release of DHT, T and E2 from luteal cells.