Japanese medaka (Oryzias latipes) were continuously exposed to concentrations of 0, 2, 20, and 50 microg/L octylphenol, 0.01 per thousand dimethyl sulfoxide, and 100 ng/L 17beta-estradiol from 2 to 4 h postfertilization until maturity. Exposure to > or =2 microg/L octylphenol caused a 20-30% increase in mortality both before and after hatch. Growth was significantly decreased in fish exposed to 50 microg/L octylphenol and in the positive control. Sex ratio changed from 42:58 (females:males) in controls to 57:40 at 50 microg/L octylphenol, and 3 and 4% of the medaka exposed to 50 microg/L octylphenol and 17beta-estradiol developed testes-ova. In cross-mating experiments, mating of exposed males with control females resulted in an up to 11% lower fertilization rate than in controls. Results indicate that continuous exposure to octylphenol causes a broad variety of reproductive effects, which can be correlated with the xenoestrogenic potential of octylphenol.