1. There are conflicting reports regarding the effect of oral contraceptives on serum and erythrocyte folate concentrations and on folate absorption. A detailed study of this problem was therefore undertaken.
2. It was first ascertained that there was no significant variation in serum folate concentrations at different stages in the normal menstrual cycle.
3. No significant difference was found in fasting serum folate concentrations between control subjects and those taking oral contraceptives.
4. Synthetic sex hormones did not inhibit jejunal pteroylpolyglutamate hydrolase in vitro.
5. Tests of absorption of pteroylglutamic acid (folic acid) and pteroylpolyglutamates in controls and women taking oral contraceptives showed no difference between the two groups, provided the subjects were presaturated with pteroylglutamic acid. Without presaturation, the rise in serum folate after oral pteroylpolyglutamates was significantly decreased in the women taking oral contraceptives, suggesting that the latter may alter folate clearance from the blood.