Ca2+ sensitivity and caffeine-induced sensitivity changes in skinned carp heart fibers were compared with those of guinea pig and rat heart. The Ca2+ concentration-response curves of saponin-treated left atrial skinned fibers obtained from guinea pig and rat were almost identical. Doses of 5 and 20 mmol.l-1 caffeine shifted this curve to the left. However, when a relatively high concentration (50 mmol.l-1) of caffeine was used, the left-ward shift was reduced. Caffeine reduced the peak of the Ca2+ concentration-response curve. The Ca2+ concentration-response curve of carp atrial skinned fiber is almost identical to that of guinea pig and rat. However, a further increase in Ca2+ sensitivity was observed even when 50 mmol.l-1 caffeine was added. Similarly, a decrease in the response curve peak was also observed. Ca2+ sensitivity in ventricular skinned fibers obtained from carp was almost the same as that observed for the atrial, but the increase in Ca2+ sensitivity due to caffeine was larger. In addition, a further increase was also observed when 50 mmol.l-1 caffeine was added. These results indicate that the Ca2+ sensitivity of contractile proteins in atrial muscles from carp heart is the same as that of guinea pig and rat. It is, however, assumed that there are some differences in properties in the contractile proteins. It is also assumed that there are some differences between the atrial and ventricular muscles of carp heart.