The authors evaluated QT interval changes after 7 and 14 days of treatment with 3 different betablockers--acebutolol, atenolol and penbutolol--in 3 groups of hypertensive patients. Acebutolol (400 mg u.i.d.) prolonged QTc interval in a statistically significant fashion, atenolol (100 mg u.i.d.) induced a significant and constant reduction, whereas QTc interval was shortened by penbutolol only after the first week, shifting toward pretreatment values during the second week. The physiological correlation between QT and RR was slightly modified by acebutolol and penbutolol, but markedly reduced after atenolol. Such differences might be of importance in clinical practice because are likely to be linked to different mechanisms intrinsic to each agent.