Myosin heavy chain isoforms of the ventricular myocardium from crucian carp (Carassius carassius L.) hearts were analyzed in different times of the year by gradient sodium dodecyl sulfate-polyacrylamide gel electrophoresis [K. A. Esser, M. O. Boluyt, and T. P. White, Am. J. Physiol. 255 (Heart Circ. Physiol. 24): H659-H663, 1988]. In winter only one myosin heavy chain type was present, but in summer about one-half of the winter myosin was replaced by more slowly moving summer myosin. The occurrence of summer myosin correlated with seasonal changes in water temperature of the pond, where the fish were caught. Furthermore, the heavy chain composition of the heart was altered by temperature acclimation in the laboratory: cold-acclimated (2 degrees C) fish had only winter myosin, but warm-acclimated (22 degrees C) fish had both summer and winter myosin in about equal amounts. Myosin adenosinetriphosphatase activity of the hearts containing both summer and winter myosin was higher than that of hearts containing only winter myosin. Functionally, changes in myosin heavy chain composition were associated with inverse thermal acclimation in the heart rate. Warm-acclimated fish had higher in vitro heart rate and shorter contraction duration than cold-acclimated animals. Present findings suggest that changes in myosin heavy chain composition together with concomitant changes in Ca2+ activation of contraction make possible large seasonal alterations in the activity of crucian carp hearts. These adjustments are needed to adapt the cardiovascular system to winter hibernation and summer activity, which are dictated by seasonally bound changes in environmental conditions.