1. The specific gravity and zinc concentration of plasma obtained during fasting were measured in pre- and post-menopausal women. A number of the latter had been taking 20–40 μg of mestranol daily for at least 1 year.
2. A highly significant correlation was found between plasma zinc concentration and plasma specific gravity in the oophorectomized women who were not receiving oestrogens.
3. The mean plasma zinc concentration was 2 μg/100 ml lower in the pre-menopausal women than in the oophorectomized controls, and this was wholly consistent with the associated fall in plasma specific gravity.
4. The mean plasma zinc concentration in the mestranol-treated women was 12 μg/100 ml lower than that found in the oophorectomized controls, and could not be attributed to the small associated change in plasma specific gravity.
5. It is suggested that the fall in plasma zinc concentration associated with mestranol therapy could be due to either a qualitative change in the plasma proteins or to relative zinc depletion.