Prolonged treatment of spontaneously hypertensive (SHR) rats with either methyldopa or clonidine significantly and similarly reduced mean arterial pressure. Heart rate increased with methyldopa but decreased with clonidine; other haemodynamic effects were similar. SHR treated methyldopa, but not clonidine, had significantly reduced cardiac masses and heart weight to body weight ratios than did rats receiving the vehicle. Normotensive Wistar Kyoto (WKY) rats treated with methyldopa also had lower cardiac masses than their controls, but exhibited no significant changes in systemic haemodynamics. Treatment with the vasodilator hydralazine increased cardiac output and decreased total peripheral resistance in both SHR and WKY, but the decrease in mean arterial pressure was significant only in SHR. Heart rate was decreased and heart weight to body weight in WKY was elevated. Cardiac mass did not change. Minimal changes in regional haemodynamics were noted, with hydralazine eliciting a decrease in most organ vascular resistances. The data indicate that regression of hypertrophy following antihypertensive therapy was not solely dependent upon haemodynamic factors; nor was it necessarily the effect of suppression of the adrenergic system, since the results obtained with methyldopa and clonidine was similar.