A human pituitary cell line (18-54, SF) grows in serum-free medium and secretes prolactin (PRL). Autoregulation of pituitary cell growth and PRL production by exogenously supplied ovine PRL (oPRL) was investigated. Human PRL (hPRL) and oPRL stimulated pituitary cell growth up to 92% and 85%, respectively, at hPRL and oPRL additions of 100-1,000 ng/ml. Short-term (1 h) incubation of the cells with oPRL decreased hPRL secretion from the cells by 72% at 10 ng/ml addition. Intracellular hPRL was stimulated under the same conditions by 50-275% at oPRL concentrations of 10-1,000 ng/ml. Long-term (10 days) incubation of the cells with oPRL had no significant effect on extracellular or intracellular hPRL production. These data suggest that the pituitary gland can serve as a primary feedback site and that PRL can autoregulate its own production as well as affect the growth of pituitary cells.