Catecholamines may be one of the molecular signals linking increased circulatory demand to myocardial hypertrophy, and I have found previously that norepinephrine stimulates hypertrophy of cultured neonatal rat heart muscle cells through an alpha 1-adrenergic receptor. Since catecholamine stimulation of contractility is believed to be under beta-adrenergic control, I asked whether these cultured heart cells had dual pathways regulating growth and contractility through alpha- and beta-adrenergic receptors, respectively. I examined the effect of adrenergic agents on hypertrophy and beating of myocytes in serum-free cultures. Hypertrophy was defined as an increase in myocyte surface area and in cell protein content, measured by a radioisotopic method, and chronotropic activity was examined visually. Norepinephrine and epinephrine were equipotent stimulants of hypertrophy and beating, increasing cell protein and area 1.5- to 2-fold, and the proportion of beating cells from 5% or less to 95%. Response maxima occurred 24-48 hours after exposure, and EC50 were 20-200 nM. Studies with other agonists (phenylephrine, methoxamine, clonidine, isoproterenol, dopamine) and antagonists (prazosin, terazosin, yohimbine, propranolol, betaxolol, ICI 118,551) indicated that hypertrophy was mediated through an alpha 1-adrenergic receptor, whereas the induction of beating required both alpha 1- and beta 1-receptor activation. Hypertrophied cells with minimal beating were produced by alpha-stimulation, alone. In contrast, alpha-plus beta-stimulation in the presence of cycloheximide to inhibit protein synthesis resulted in maximum beating but no hypertrophy. These findings imply that growth and beating can be regulated independently through separate cellular pathways.