Although zebrafish (Danio rerio) have been suggested as a good candidate for screening potential endocrine disruptors, little information is available on the effects of weak estrogens on this species. We investigated the sensitivity of different life stages of zebrafish toward toxic and estrogenic properties of methoxychlor (MXC). Short-term tests with adults resulted in a sex-specific 96-h lethal concentration for 50% (LC50) of the test animals of 36 microg/L for males and 129 microg/L for females. To determine the estrogenic capacity of MXC, adult zebrafish were exposed to 0, 0.5, 5, and 50 microg MXC/L for 14 d. Induction of vitellogenin ([VTG] measured with protein electrophoresis and Western blot) in males was detected at 5 and 50 microg MXC/L. Females, however, did not exhibit higher blood VTG concentrations at the tested MXC concentrations. In a second series of experiments, juvenile zebrafish were exposed to 0, 0.05, 0.5, and 5 microg MXC/L for 33 d. Survival, length, weight, and condition of larvae were examined as indicators of toxic stress and the VTG content in whole body homogenates of juveniles was measured to determine xenoestrogenic effects. No effects of the tested concentrations of MXC were observed. Finally, the effect of MXC on zebrafish eggs, exposed to 0, 1, 10, and 32 microg MXC/L, was examined. Hatching and survival of hatched zebrafish were affected at 10 and 32 microg MXC/L. This study demonstrated that adult male zebrafish are sensitive toward the estrogenic effects of MXC. However, the use of VTG induction to detect effects of (xeno)estrogens in early life stages has to be further investigated, as low concentrations of VTG were detectable in exposed as well as unexposed juvenile fish.