Six healthy young men participated in a confined zinc depletion-repletion study which lasted 10 to 11 wk. The depletion period varied from 4 to 9 wk during which the subjects consumed a semipurified formula diet supplying 0.28 mg zinc/day. During a 1-wk initial baseline period, 15.7 mg of zinc as ZnSO4 were fed; 6.0, 23.2, or 46.3 mg of zinc were given during a brief repletion period for three of the men. Plasma, whole blood, urinary, fecal, and seminal zinc decreased significantly by the end of the depletion period. Erythrocyte, saliva, and hair zinc did not change. Plasma and urinary zinc levels were highly correlated when urinary zinc was 150 micrograms/day or more. In five of six subjects a drop in urinary zinc to below 150 micrograms/day was noted before a decrease in plasma zinc to less than 70 micrograms/100 g, indicating that urinary zinc responds more rapidly than plasma zinc to dietary changes and may be useful for evaluating zinc nutriture. The endogenous zinc loss averaged 2.2 mg/day; addition of the loss occasioned by one seminal emission would bring the total to 2.8 mg/day.