The relative estrogenicity of nonylphenol and its ethoxylates has not been clearly demonstrated in the literature, despite the importance of this information for interpreting the environmental risk of these chemicals. There appears to be a discrepancy between the relative acute/chronic toxicity and estrogenicity reported in previous studies. These studies have suggested that the relatively higher concentrations of nonylphenol polyethoxylate metabolites (NP n EO, NP n EC) in municipal effluents may represent a risk to the environment. However, there is considerable uncertainty associated with the estimates of relative estrogenicity of these metabolites.Plasma vitellogenin (Vg) was measured in rainbow trout (Oncorhynchus mykiss) after a 21-d flow-through exposure to concentrations of 1-250 µg · L −1 nonylphenol (NP), 1-280 µg · L −1 nonylphenol 1-ethoxylate (NP1EO), or 24-1450 µg · L −1 nonylphenol 1-ethoxycarboxylate (NP1EC). All three chemicals induced plasma Vg to varying degrees, their relative estrogenicity being NP > NP1EO >> NP1EC. Measurements of the relative potency of NP1EO and NP1EC compared to NP, yielded ratios of 0.22 and 0.03, respectively. These values are in general agreement with relative acute and chronic toxicity data in the literature. A re-evaluation of the estrogenicity of the biodegradation products of nonylphenol polyethoxylates in Canadian sewage treatment plant effluents was performed, using the relative estrogenicity determined in this study, and revealed that the contribution of alkylphenol polyethoxylates to effluent estrogenicity is significant but less than previously estimated. The ability of these chemicals, however, to act in concert with other estrogenic compounds such as 17β-estradiol, estrone, and 17α-ethinylestradiol, to provide a cumulative estrogenic exposure for the biota needs to be investigated.