Environmental contaminants have been identified as endocrine disruptors through their antiandrogenic activity. Thus, as androgen receptor antagonists, the fungicide vinclozolin and the principal DDT metabolite p,p'-DDE have been demonstrated to induce demasculinization in rats. Whether this is also the case in fish remains to be demonstrated. For a period of 30 days, groups of adult male guppies were exposed to vinclozolin, p,p'-DDE, or the therapeutic antiandrogen flutamide (used as positive control) applied to the fodder at concentrations between 0.1 and 100 microg/g fodder. Subsequently, sexual characteristics of relevance to the male reproductive capacity were measured and compared with untreated control fish. All three chemicals caused profound alterations at increasing levels of biological organization, even in these fully matured males. At the cellular level, the three compounds induced a significant reduction in the number of ejaculated sperm cells. At the organ level, the sexually attractive orange-yellow coloration was reduced in area and discolored, and treated fish also had smaller testes. Further, at the organismal level, computer-aided behavior analyses demonstrated a severe disruption in male courtship behavior. We conclude that this demasculinization is consistent with an antiandrogenic action of vinclozolin and p,p'-DDE and is likely to compromise reproductive capability in this fish.