Long-Evans rat pups were dosed orally from birth to 21 days with particulate Mn3O4 to obtain a daily dose of 0, 71 or 214 micrograms Mn/g BW/d. Assessment of the hypothalamic, pituitary or testicular functions were determined by measuring the endogenous or stimulated serum concentrations of follicle stimulating hormone (FSH), luteinizing hormone (FSH), luteinizing hormone (LH) and/or testosterone (T) at 21 or 28 days of age. Body, testes and seminal vesicles weight and tissue concentrations of Mn treatment effects were seen in body and testes weights. No effects were seen either on unstimulated or stimulated FSH or LH serum concentrations. Although no Mn treatment effects were seen on endogenous or two hour hCG stimulated serum T concentrations, there was a reduction in the serum T following seven days of hCG stimulation. The hypothalamic Mn concentrations in animals with these reproductive effects were three times those where alterations in the dopaminergic pathway have been reported.