The reproductive success of most fish depends partly on their ability to perform correct courtship behaviour. Alteration of this behaviour due to xenooestrogens can potentially affect the reproductive output and consequently population dynamics. In this lifetime study, zebrafish (Danio rerio) were exposed to environmentally realistic (0.05, 0.50 and 5.00 ng L(-1)) nominal concentrations of 17alpha-ethinyloestradiol (EE2) in a flow-through system for 4 months, from egg until sexual maturity. At 0.05 ng EE2L(-1), the secondary sexual characteristics of the males were significantly feminised with development of urogenital papillae and change in body colour. At 0.50 ng EE2L(-1), the sex ratio was altered from 69% males in the control groups to 59% males. The courtship behaviour of the male zebrafish was not affected by these two lower EE2 concentrations. Only at the highest concentration of 5.00 ng EE2L(-1), where only 5% of the group developed into males, was a change in behaviour recorded, and these few males were unable to induce spawning. Accordingly, the courtship behaviour of zebrafish is more resistant to oestrogenic exposure than secondary sexual characteristics and gonad development. This study provides the first quantitative measure of zebrafish courtship behaviour.