Involvement of cholesterol in thermally induced restructuring of biological membranes was investigated in several tissues of rainbow trout (Oncorhynchus mykiss). Cholesterol-rich plasma membranes (PM) were isolated from erythrocytes, liver, kidney, and gill of fish acclimated to 5 and 20 degrees C. Mean PM cholesterol-to-phospholipid molar ratios (C/P) from warm-acclimated animals were significantly higher than those of cold-acclimated fish in liver (0.26 vs. 0.18; P < 0.01), kidney (0.49 vs. 0.40; P < 0.02), and gill (0.66 vs. 0.60; P < 0.05); erythrocyte C/P did not differ significantly with acclimation temperature (0.28 vs. 0.25; P = 0.25). In light of the ordering effects of cholesterol on fluid-phase membranes, these results are consistent with a role for cholesterol in the homeoviscous response of some poikilotherm PMs. Tissue differences in both PM cholesterol levels and the magnitude of thermally evoked cholesterol changes may reflect tissue-specific membrane functions. Lower PM C/P of trout tissues relative to corresponding data available for homeotherms also support a possible evolutionary relationship between cholesterol content and thermal adaptation of the PM.