Serum prolactin was measured in single blood samples collected from 219 nursing mothers of the Kivu region (Zaïre) during 30 post-partum months. In addition the number of feeding episodes per day and the amount of milk given to the child in 24 h were recorded. The mean serum prolactin levels remained around 1000 mu/l during the first 15 months of lactation and fell during the next 3 months to 550 mu/l. A decline in milk production per day occurred during the second year, but it was less marked than that of prolactin. This decline seemed to be associated with the decline in suckling frequency as the quantity of milk given per feed remained almost unchanged throughout lactation. The average amount of milk given by mothers with serum prolactin levels in the range of values seen in non-lactating and non-pregnant women (about 500 mu/l) is nevertheless of some 35 g per feeding or 260 g per day. These results demonstrate that milk production can be maintained in women with normal levels of prolactin and suggest that prolactin plays a permissive role in established lactation.