Sprague Dawley male rats (200-220 g) were subjected to a long-term treatment with low doses of catecholamines in an attempt to assess the effect of the adrenergic stimulation on ventricular weights, hemodynamics, and hydroxyproline content in the myocardium. Data are presented indicating that both exogenous catecholamines (i.e., isoproterenol) and endogenous catecholamines (released from tyramine) are able to bring about a degree of myocardial hypertrophy associated with a significant increase in the ventricular concentrations of hydroxyproline, without affecting hemodynamic parameters (blood pressures and heart rate). Thus the modulation of sympathetic tone in the genesis of cardiac hypertrophy is demonstrated, and a direct effect of the catecholamines on cardiac cells is postulated which is independent of hemodynamic changes.