Zinc deficiency was induced in two species of monkeys, Macaca mulatta (rhesus) and Macaca radiata (bonnet), by feeding a purified diet containing isolated soybean protein. In both rhesus and bonnet monkeys, plasma zinc levels were reduced in 7 or 14 days after institution of the zinc-deficiency regimen. Dermal lesions on the extremities, face and abdomen, and alopecia also appeared in both species. Reduced hair zinc concentration was observed in zinc-deficient bonnet monkeys. Oral supplementation with zinc rapidly reversed the dermal lesions and hair loss of deficient monkeys, however, repleted monkeys still had lower hair zinc levels than controls. Normal reproduction was impaired when the zinc-deficient diet was fed to adult monkeys. In pregnant rhesus monkeys, one zinc-deficient monkey aborted on day 48 of pregnancy. Under other experimental conditions, fetuses removed by cesarean section on day 70 or 100 of pregnancy did not show gross congenital abnormalities. In zinc-deficient bonnet monkeys, none of the matings resulted in pregnancies, and after prolonged deficiency menstrual cycles ceased together. The zinc-supplemented control diet supported normal reproduction.