The changes induced in prolactin levels during the first 6 days following delivery were studied. Blood samples for prolactin assay were taken from 9 women at 5-min intervals during breast feeding. The levels of lactation were followed via the amounts of milk produced during suckling. The highest prolactin levels were observed on days 2-4 following delivery. There appeared to be some correlation between the basal and breast-feeding-induced prolactin levels and the level of lactation. A relatively low basal level and a moderate feeding-induced response are early indicators of delayed and less productive lactation. In unfavourable cases the feeding-induced prolactin increases gradually disappeared and lactation stopped. The correlation between the prolactin level and the quantity of milk formed is not a close one, but the observed tendencies agree with the role of prolactin. From day 5 on, the prolactin demands of the breasts are lower.