1. Serum prolactin (PRL) levels, both basal and post-suckling peak, were estimated in fifty-seven lactating women.
2. Basal PRL levels were significantly higher in all lactating women irrespective of the duration of lactation as compared to the levels in non-pregnant, non-lactating women.
3. There was a significant positive correlation (r 0·69, P < 0·001) between the basal PRL levels and the post-suckling peak values.
4. After 8 months of lactation, peak post-suckling, PRL levels were not significantly different from basal values.
5. Basal PRL levels were significantly lower (P < 0·05) in mothers whose infants were being supplemented.
6. PRL levels were not related to the mother's nutritional status as determined by body-weight.
7. The findings suggest that high PRL levels during lactation prolong the duration of lactional amenorrhoea and hence the relative infertility.