The effect of adenosine compounds (ATP, ADP, AMP, and adenosine) on membrane potential, current and contractile tension on the bullfrog atrium were studied under voltage clamped and unclamped conditions. The compounds produced immediate positive and late negative inotropic effects in unclamped conditions. The positive isotropic effect and the potency of drugs appeared less marked in the order of ATP, ADP, AMP, and adenosine. Under voltage clamped conditions, only the energy rich compounds, ATP and ADP, produced an enhancement of calcium inward current (I_Ca) and I_Ca-dependent phasic tension, while AMP and adenosine elicited a negative inotropic effect. The delayed outward current was initially depressed but later augmented especially in case of ATP and ADP where I_Ca was enhanced. All adenosine compounds, however, inhibited the I_Ca-independent tonic tension. This effect, appearing nonspecific, was ascribable to the action of common structure of these compounds, purine-riboside moiety.