alpha 1-Adrenergic receptor-mediated responses are overwhelming in adult rat hepatocytes. Inversely, beta-responses are predominant over alpha 1-responses in the hepatocytes that have been cultured at a low cell density (10(4) cells/cm2) for 24 h. The insulin-EGF-induced DNA synthesis in the beta-response-dominant hepatocytes was doubled by beta-agonists or cAMP-generating agents added far behind (16-20 h) the addition of insulin/EGF; i.e., immediately before the entry into the S-phase of the cell cycle. Agonists of alpha 1-adrenergic or other Ca2+, mobilising receptors added to the alpha 1-response-dominant hepatocytes increased DNA synthesis only if they were added within 1-2 h after the addition of insulin/EGF, at the early stage of G1-phase. Agonists of "non-dominant" receptors were rather antagonistic to agonists of "dominant" receptors. Thus, agonists of alpha 1-adrenergic (and other Ca2+ mobilising) receptors and agonists of beta-adrenergic (and other cAMP-generating) receptors acted as comitogens in their own particular manners in the presence of growth factors in hepatocytes in which the respective receptor functions were dominant.