The PRL response to suckling was studied during the first week of the puerperium. Mean basal levels of PRL showed no significant change during the first week of the puerperium, but there were progressive rises in both the maximum suckling-induced response and the total area under the response curve, which reached peak values on the fourth day after delivery. Despite large variations between individuals in basal PRL levels (range, 0.3-7.0 U/liter), peak suckling-induced response (range, 0.1-9.9 U/liter), and total response (range, 0.6-63.0 arbitrary units), there was much less variability within individuals between consecutive feeds. Using an electronic balance, 20 patients on days 5 and 6 were classified either as good feeders (>60 g milk/feed) or poor feeders (≤60 g milk/feed) on the basis of 2 consecutive test weights. The mean PRL response to suckling in 11 good feeders was no different from that in 9 poor feeders, and there was no significant correlation between milk yield and PRL response. Six patients whose infants were in the special care nursery had lactation initiated and maintained by breast pump for an average of 5.6 days. Although the PRL response to the breast pump was very small, these patients also had satisfactory milk yields (mean, 86 g). Although the presence of PRL is essential for lactation, the data in this paper suggest that there is no close teriporal correlation between PRL concentrations and milk yield.