Retinol-binding protein, prealbumin, and sex steroid plasma levels have been estimated daily in four women during the course of a normal menstrual cycle and in three women during treatment with combined oral contraceptives. The retinol-binding protein level showed a bicyclic variation during the menstrual cycle, whereas for prealbumin no consistent pattern of variation was observed. Oral contraceptive therapy induced a significant increase of retinol-binding protein which was correlated with the increase of vitamin A. The increase was about 35% for the formulation containing synthetic estrogen as compared with 15% for the one with natural estrogen. In relation to retinol-binding protein, the plasma level of prealbumin showed a less rapid increase but the final increment after oral contraceptive therapy appeared to be of the same magnitude for both proteins.