Calcium ionophore A23187 being added at 2.5 mM concentration induced 2--3-fold increase of peak contractile force of isolated guinea pig papillary muscle stimulated with the frequency of 0.2 Hz. The increase of force of contraction is not followed by any change in amplitude and duration of membrane action potential. A23187 decreases testing tension as well as duration of the twitch and increases relaxation rate of the muscle. This phenomenon probably reflects increased capability of sarcoplasmic reticulum in the process of calcium ion accumulation. In the presence of ionophore appearance of shoulder on the rising part of the twitch has been registered. In this case the single twitch being differentiated reveals two positive maxima. Addition of D-600 leads to decrease of action potential duration and eliminates the second part of the twitch, the amplitude and the velocity of the first phase of contraction being without change. It was proposed that the first component of the twitch appearing in the presence of A23187 and insensitive to D-600 corresponds to the release of calcium from some intracellular stores.