125I-Labelled ovine prolactin was infused for 15 min into a pudic artery supplying one mammary gland of lactating goats (n=17). Between 0 and 4·25 h significandy more total (P<0·01) and trichloroacetic acid (TCA)-precipitable (P<0·001) radioactivity appeared in the milk of the infused compared with the non-infused gland. Gel chromatography and antibody precipitation indicated the presence of undegraded 125I-labelled prolactin in milk whey. Maximum transfer occurred 60–80 min after the end of infusion suggesting passage via a transcellular route. High plasma prolactin concentrations, resulting from infusion of cold prolactin with labelled prolactin in late lactation or from seasonally elevated prolactin at peak lactation, reduced the specific activity of infused prolactin and depressed the difference in secretion of 125I-labelled prolactin into milk of infused and non-infused glands. This suggests the operation of a competitive and saturable mechanism. Together with the increase in the milk to blood ratio of prolactin in goats given long-term (3 week) bromocriptine treatment, the results suggest that the goat mammary gland has a high avidity for prolactin especially when circulating prolactin is low. There was also evidence from TCA precipitation that prolactin may be protected from degradation in these circumstances. These mechanisms may contribute to the resistance of ruminant lactation to reduction in plasma prolactin and protect lactation from seasonal prolactin fluctuations.