1. Following acclimation of channel catfish to a reduction in temperature from 25 degrees to 15 degrees C, there were approximately two-fold increases in liver mass, cell size, total protein, and total enzyme activity, relative to activity per milligram of protein and per gram wet weight of tissue, indicating tissue hypertrophy. There was no change in either total liver DNA content or protein concentration per gram weight. 2. Green sunfish, unlike catfish, showed virtually no change in liver mass following cold acclimation. However, sunfish showed a net increase in total liver protein content and an increase in protein concentration. The increase in protein content was balanced by a reciprocal and equivalent decrease in glycogen content. Consequently, liver mass was maintained. 3. During cold acclimation both catfish and sunfish showed an increase in ventricular heart mass and protein content, but no change in protein concentration. 4. The activities of several enzymes were measured in liver from 15 degrees C and 25 degrees C steady-state-acclimated catfish and at intervals following transfer from 15 degrees to 25 degrees C and from 25 degrees to 15 degrees C. Total tissue enzyme activity showed positive compensation which correlated with the change in liver mass and protein content. Specific activities based on protein and on wet weight showed dissimilar acclimatory patterns. Two enzymes - cytochrome oxidase and lactate dehydrogenase - showed inverse compensation in specific activity but positive compensation in total activity. Citrate synthase, glucose-6-phosphate-dehydrogenase and 6-phosphogluconate dehydrogenase showed positive compensation in both specific and total activities. 5. The increase in tissue protein content or 'protein hypertrophy' occurred with cell hypertrophy in cold-acclimated catfish, while protein hypertrophy occurred as an increased protein concentration without cell hypertrophy in sunfish. This phenomenon is considered adaptive in that it permits a compensatory increase in the total enzymatic capacity of a tissue. The two-fold increases in total enzyme activities, superimposed on either an increase or decrease in specific activity, suggest that two biochemical mechanisms may be operative during cold-induced liver hypertrophy, one effecting a specific step in protein translation at a point common to the synthesis of all proteins and a second targetted pretranslationally, i.e., transcriptional regulation.