Summary The effect of hyper-and hypotonic solutions on the electrical as well as the mechanical activities of the bullfrog ventricle were investigated. Hypertonic solutions up to 400 % tonicity, made by adding sucrose, showed two kinds of inhibitory effect; i.e. a consistent, rapid shortening of the action potential duration and a slowly progressing suppression of contractile tension. The inhibitory effect of the hypertonic solution up to 300 %, made by adding NaCl, was more pronounced on the contractile process than on the electrical phenomena. In this condition, the inhibition of the twitch tension occurred with two time constants. The first, rapid decline seems to be related to the suppressive effect on some superficial site while the slower decreasing phase showed a similar time course as the development of contracture. Hypotonic (33.3 % and 12.5 %) perfusion revealed triphasic changes in the time course of single twitch without accompanying any parallel change in the action potential. From these results it was concluded that the bullfrog ventricular muscle has some superficial site for Ca regulation which is very sensitive to osmolarity change.It is known that the twitch contraction of skeletal muscle can be strongly suppressed in hypertonic media without any essential change in the action potential (HODGKIN and HOROWICZ, 1957;HOWARTH, 1958). It has been shown that the caffeine-induced contracture is also inhibited by hypertonic perfusion (ISA-ACSON, 1969). The mechanism of the contractile inhibition by hypertonicity is still unknown although a direct suppressive effect on the contractile elements is postulated by many authors.Recently LANNERGREN and NOTH (1973) have reported that the hypertonic