Male weanling rats were fed four diets providing high or low levels of protein and/or zinc. One group on each diet was sacrificed every 2 weeks to measure the weights of the reproductive organs, plasma levels of follicle-stimulating hormone and luteinizing hormone, and the plasma and testicular concentrations of testosterone, dihydrotestosterone and zinc. The results demonstrate that hypogonadal states are produced in response to protein deficiency and zinc deficiency but the mechanisms involved are different. The specific effects of zinc deficiency are not observed in protein-deficient animals.