Long-Evans rats were chronically exposed to dietary manganese beginning on d 1 of gestation and continuing through 224 d of age. Dietary concentrations of manganese were 350, 1050, and 3500 ppm and were applied in either normal Fe 240 (ppm) or a low-Fe (20 ppm) basal diet. General toxic effects were apparent in young animals at a dietary dose of 3500 ppm Mn and were enhanced by concomitant Fe deficiency. Fertility was reduced in the group exposed to 3500 ppm Mn with a diet containing sufficient Fe. Male reproductive development was delayed by Mn treatment, as measured by testes weight, sperm count, and serum follicle-stimulating hormone and testosterone concentrations.