The effects of exogenously applied adenosine on the membrane potentials, currents and tension components of the bullfrog double sucrose-gap method. Adenosine above 10(-3)M produced a prolongation of action potential accompanied by a slight augmentation of contraction which was followed by a sustained depression. The recovery was slow. Under voltage clamp conditions, adenosine merely produced a negative inotropic effect depressing Ica-dependent and -independent tensions. In the membrane currents, Inaf, Is(Ica), Ix and background current (Ib) were all depressed. In the presence of adrenaline, the increased Is, Ix and contraction were inhibited by adenosine in a lower dose (10(-5) -10 (-3) M), and the inhibition of Is and Ica-dependent tension were more prominent in the presence of than in the absence of adrenaline. Under the effect of ATP, which has a catecholamine-like action, similar selective inhibitions were observed. It was concluded that adenosine has a sustained stabilizing action on the myocardium to survive, especially in the presence of adrenaline, by depressing the augmented Ica and contraction.