Rainbow trout (Salmo gairdneri), average body weight of 450 g, were treated with 15, 25, or 35 mg/kg of ciprofibrate via intraperitoneal injection every other day for 2 to 3 weeks. The effects on hepatic peroxisomal acyl-CoA oxidase, polypeptide PPA-80, catalase, and liver weight were measured. The treatment of trout with ciprofibrate showed significant dose-related increases in peroxisomal acyl-CoA activity, polypeptide PPA-80, and catalase after 3 weeks of exposure. Peroxisomal oxidase activity showed a significant (p = 0.0008) increase (78%) at 35 mg/kg and a marginal (p = 0.1) increase (27%) at 25 mg/kg after 3 weeks of exposure. Densitometric analysis of polypeptide PPA-80 and catalase showed increases up to 48 and 236% at 35 mg/kg, respectively. Morphometric analysis on livers of trout administered 35 mg/kg for 3 weeks showed a 2.3-fold increase of peroxisomal volume density, as compared to control. This study demonstrates the induction of peroxisome proliferation in rainbow trout administered ciprofibrate, a known peroxisome proliferator in rodents.