The influence of selective (milrinone: 10, 50, 100 μM) and nonselective phosphodiesterase (isobutyhnethylxanthine: 0.1, 10, 100 μM) inhibitors and β-adrenergic stimulation (isoproterenol: 0.01, 0.1 μM) on phospholamban and myofibrillar protein phosphorylation was studied in guinea pig hearts perfused with [32P]orthophosphate. Changes in protein phosphorylation were compared to alterations in tissue cyclic AMP (cAMP) levels and positive inotropic effects induced by these agents. Isoproterenol (0.01 μM), milrinone (50 μM), and isobutyhnethybumthine (100 μM) all produced similar, twofold increases in dP/dt and -dP/dt but only stimulation with isobutylmethylxanthine and isoproterenol was associated with significant increases in phospholamban phosphorylation. At these equipotent doses, the effects of isobutyhnethylxanthine were associated with higher increases (3.1-fold) in cAMP than those observed with isoproterenol (twofold). Milrinone (50 μM) produced a 2.5-fold increase in cAMP levels but failed to change phospholamban phosphorylation. Higher doses of milrinone (100 μM) resulted in relatively high (4.1-fold) cAMP levels, and this was associated with increased (1.5-fold) phosphorylation of phospholamban. Phosphorylation of troponin I was significantly increased at 0.01 μM and 0.1 μM isoproterenol, while phosphorylation of C protein was observed only at 0.1 μM isoproterenol. Isobutyhnethylxanthine and milrinone did not significantly increase phosphorylation of either troponin I or C protein at any of the doses studied. These findings indicate that cardiotonic agents acting via the cAMP pathway may produce similar inotropic responses at different levels of cAMP and phosphorylation of sarcoplasmic reticulum and myofibrillar proteins.