The effects of the synthetic estrogen 17alpha-ethinylestradiol (EE) on sexual development of the freshwater amphipod Hyalella azteca was investigated. Organisms were exposed in a multigeneration experiment to EE concentrations ranging from 0.1 to 10 microg/L and the development of both external and internal sexual characteristics were studied. Second-generation male H. azteca exposed from gametogenesis until adulthood to 0.1 and 0.32 microg EE/L developed significantly smaller second gnathopods. The sex ratio of the populations exposed to EE for more than two generations tended, although not statistically significantly, to be in favor of females. Histological aberrations of the reproductive tract, i.e., indications of hermaphroditism, disturbed maturation of the germ cells, and disturbed spermatogenesis, of post-F1-generation males were observed in all EE exposures. These findings provide evidence that sexual development of H. azteca is affected by exposure to sublethal concentrations of EE.

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