Summary: In the canine heart–lung preparation (HLP) and the anesthetized open-chest dog, both 8-(Benzylthio)-N6-n-butyl–cyclic AMP (BTB–cyclic AMP) (10 times more potent) and dibutyryl–cyclic AMP (DB–cyclic AMP) produced a definite positive inotropic effect (PIE) and an increase in the coronary blood flow with either no change (HLP) or a slight increase (anesthetized animal) (BTB–cyclic AMP) and a definite increase in the heart rate (DB–cyclic AMP). In the isolated atrial preparations of the guinea pig (AG), BTB–cyclic AMP produced a slight PIE at 3 × 10−5 M and a negative chronotropic effect at 3 × 10−4 M. Aminophylline reversed the latter to the positive one, while potentiating the former. At 10−3 M marked positive inotropic and chronotropic effects were observed. DB–cyclic AMP was without effects. In the right ventricular papillary muscle preparations of the guinea pig (PMG), both compounds produced PIE. BTB–cyclic AMP initiated a contraction (AG) and a “slow” action potential (PMG) in partially depolarized preparations. Both of these effects were abolished by diltiazem. BTB–cyclic AMP (10−3 M) suppressed the heart rate increase induced by isoproterenol in right AG. It was concluded that BTB–cyclic AMP was a cardiotonic agent with a relatively weak positive chronotropic effect and that the cardiotonic effect was ascribable to the initiation of slow action potentials and related contractions.