Technological innovations such as modified and extended kraft cooking, improved chemical recovery, reduced water usage, elementa- chlorine-free bleaching and biological treatment of effluents have been widely implemented by the Canadian pulp and paper industry. This survey was undertaken to assess the mixed function oxygenase (MFO)-inducing potential of effluents discharged by mills at the current state of operating technology. Between December 1993 and June 1994, 46 effluent samples were obtained from 33 different pulp and paper mills. In the laboratory, rainbow trout (Oncorhynchus mykiss) were exposed to a 10% (v/v) concentration of each effluent for 96 h. Hepatic ethoxyresorufin-o-deethylase (EROD) activity was assayed in effluent-exposed and control trout. Overall, 17 of the 46 samples caused no statistically significant increases in EROD activity, 10 samples caused statistically significant but ≤2-fold EROD inductions, while the remainder of the samples caused up to 15.5-fold induction. The EROD-inducing potential of effluents could not be linked to a particular bleaching or pulping process. For example, increased MFO activity was measured in trout exposed to some effluents from both unbleached and bleached kraft mills as well as thermomechanical and chemithermomechanical mills. Presently, the precise combinations of operating conditions that control the EROD-inducing potential of effluents from these mills are unclear.