1. The cellular effects of alpha 1-adrenoceptor stimulation by phenylephrine were studied in the presence of propranolol in single cells isolated from the ventricles of rat hearts. 2. Phenylephrine (10-100 microM) induced a biphasic pattern of inotropism in these cells: a transient negative followed by a sustained positive inotropic effect as usually observed in cardiac tissues. 3. In Snarf-1-loaded cells, phenylephrine induced an alkalinization. This effect was reversible on wash-out and inhibited by prazosin, an alpha 1-adrenoceptor antagonist. 4. The alpha 1-adrenoceptor-mediated increase in intracellular pH (pHi) was 0.1 pH unit in HEPES buffer containing 4.4 mM-NaHCO3 and in Krebs buffer containing 25 mM-NaHCO3. 5. The alkalinization was blocked by the Na(+)-H+ antiport blocker, ethylisopropylamiloride (EIPA). 6. The recovery from an acidosis induced by a NH4Cl pre-pulse was accelerated by phenylephrine. The phenylephrine-induced alkalinization was attributed to activation of the Na(+)-H+ antiport. 7. Despite its ability to increase pHi, phenylephrine did not alter Ca2+ current amplitude and kinetics. 8. Ca2+ transients recorded in Indo-1-loaded cells were not augmented by phenylephrine. Diastolic calcium level was decreased. 9. In single skinned cells, the Ca2+ sensitivity of the contractile proteins was increased by a pre-treatment with phenylephrine even when the alpha 1-adrenoceptor-mediated alkalinizing effect had been prevented by EIPA. 10. These results lead us to propose that the alpha 1-adrenergic-induced positive inotropic response of heart muscle could result from an increased sensitivity of the myofilaments to Ca2+ ions. This alpha 1-adrenoceptor-mediated Ca2+ sensitization could result both from an intracellular alkalinization and from a direct effect on contractile proteins.