Zinc excretion under conditions of negligible zinc intake (0.17 mg/day) was measured in women taking a combination oral contraceptive agent (+OCA) and in women with normal menstrual cycles (-OCA). A semipurified, constant formula diet, providing negligible amounts of zinc, copper and iron, but adequate levels of other essential nutrients, was fed. During the 35--day study, serum and urinary zinc declined more markedly in the +OCA than -OCA groups. Serum zinc dropped 47% in the +OCA and 21% in the -OCA; urinary zinc declined 83% and 62% in the two groups. After pre-study, zinc was cleared from the gut, fecal zinc decreased about 40% in both groups. Whole body integumental zinc losses were similar for both groups, about 0.7 mg/day. For -OCA, zinc losses via this route were higher during the luteal phase than during menstruation. Menstrual fluid zinc contents were negligible and similar for both groups, about 5 microgram/day. The data suggest that accessible zinc stores are not extensive and that depletion of these stores, as a result of the low-zinc diet, caused the fall in serum zinc. The use of OCA influenced the response to the low-zinc diet, but endogenous zinc losses calculated for day 1 were nearly the same for both groups, about 1.6 mg/day.