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Estradiol-17P (E,) and rhe two catecholestrogens 2-OHE, and 4-OHE2, when daily administered at low doses of 10-40 ng/rat, were cytotoxic to the seminiferous epithelium. The structural changes seen after seven days exposure included abnormal meiotic type I1 cells with uneven chromosome distribution, the formation of binucleated and multinucleated giant cells, of which many were sloughed into the lumina of the seminiferous tubules. The effect of the 4-OHE, metabolites were always more pronounced than that of 2-OHE, or E,. After 21 daily exposures, 4-OHE2 proved to be very toxic, the seminiferous tubules were markedly denuded and numerous giant cells were present in the lumina. The catecholestrogens also caused a significant lowering ( P < 0.02) of testosterone serum levels after eight days exposure. E, at 40 ng/rat/day had no effect on testosterone production. At these low doses the catecholestrogens did not affect gonadotropin release after eight days exposure. Our results indicate that the morphological lesions could not exclusively be attributed to testosterone withdrawal and that a direct effect on developing spermatids is also indicated.