Groups of adult male rats were treated with DDT, DDVP or malathion orally for two or three weeks. Body weight and testis weight did not change. FSH and LH levels in serum also remained unchanged, whereas that of testosterone was decreased by DDT. This substance also diminished the concentration of testosterone in the testis. Therefore, the previously observed damage to spermatogenesis by DDT are assumed to be caused by a lack of androgens. Those following treatment with DDVP or malathion seem to be due to a direct cytotoxic effect.