White perch (Morone americana), yellow perch (Perca flavescens), and smallmouth bass (Micropterus dolomieui) were acclimated to 5 and 20 °C. There was an increase in ventricle mass relative to body mass in smallmouth bass only following acclimation to 5° C. Maximal in vitro activities of hexokinase, citrate synthase, carnitine acyl CoA transferase (with palmitoyl CoA, palmitoleoyl CoA, and oleoyl CoA as substrates), and total ATPase were assessed in crude heart homogenates. Tissues removed from warm-acclimated animals were tested at 20 and 5 °C; tissues removed from cold-acclimated animals were assessed at 5 °C. Acute temperature transitions were associated with decreases in the activities of hexokinase (Q10 ≈ 1.8), citrate synthase (Q10 ≈ 1.4), and ATPase (Q10 ≈ 1.7). The impact of temperature on carnitine acyl CoA transferases was generally less severe. This suggests that maximal fatty acid oxidation is conserved better than glucose oxidation during a warm to cold transition. Maximal enzyme activities were generally unaffected by the acclimation regime, with the exception of that of carnitine acyl CoA transferase in white perch heart. The substantial increase in carnitine acyl CoA transferase activity when unsaturated CoA derivatives were provided as substrate suggests an increased capacity to oxidize unsaturated fatty acids at low temperature following an acclimation period. Attempts to sustantiate this contention by offering labelled oleic acid to ventricle sheets were thwarted by a high rate of incorporation into the total lipid pool.