Red cell folate concentration was used to measure folate status in 89 rural Gambian women, 81 of whom were pregnant or lactating for the duration of the study. During pregnancy, the women received a haematinic supplement of 500 micrograms pteroyl glutamic acid and 47 mg iron a day, and a food supplement was also given to some of the women during either pregnancy, or lactation, or both. During pregnancy, the red cell folate levels reached a peak around the fifth month, attributable to the haematinic supplement. By the third month of lactation they had fallen back to a low plateau, which was below the accepted lower limit of normality in 56 per cent of those who did not receive a food supplement during lactation. In addition to these changes, there was a significant cyclical variation with season, peak values occurring in the rainy season. Plasma iron values followed the red cell folate patterns quite closely. Changes in blood haemoglobin levels during pregnancy were similar to those reported from other communities; there was also an increase in mean red cell volume during the first half of pregnancy, reaching a peak at the same stage as the peaks of red cell folate and plasma iron. There was no indication of the presence of raised cell volumes in association with low folate indices which might suggest megaloblastosis. It is concluded that the 500 micrograms daily folate supplement is both necessary and adequate to maintain red cell folate levels during pregnancy in this community, but that in the absence of further supplements during lactation, red cell folate levels will fall to undesirably low levels in many subjects by the third-to-sixth month of lactation.