The purpose of the study was to compare the effects of oxprenolol, a beta-adrenergic blocking agent with intrinsic sympathomimetic activity, to propranolol, a beta-blocker without intrinsic sympathomimetic activity, on myocardial performance and myocardial O<sub>2</sub> consumption (MVO<sub>2</sub>). Myocardial performance was reflected by the systolic time intervals (STI) and MVO<sub>2</sub> by the triple product of heart rate × left ventricular ejection time × systolic blood pressure (HR × LVET × SBP). The trial, a double-blind cross-over study, included 32 hypertensive patients, 17 males and 15 females, with a mean age of 50.9 years. The patients were divided into two groups: group A – 16 patients who were treated with effective antihypertensive doses of oxprenolol and diuretics (Esidrex K) for the first 8 weeks (period I) and with similar doses of propranolol and diuretics for the following 8 weeks (period II); Group B – the same number of patients who received the same drugs in the reverse order. All patients were examined during both periods at 2- and 3-week intervals, blood pressure and heart rate were recorded, and patients were provided with the appropriate study medication. At the end of periods I and II, myocardial function was assessed on the basis of STI, and MVO<sub>2</sub> was evaluated on the basis of the triple product of HR × LVET × SBP. Four measurements of STI and MVO<sub>2</sub> were carried out for each patient at the end of periods I and II: prior to intake of medication and 1½, 3 and 4½ h after intake of medication. The results of the study indicated in all four measurements a tendency for lower myocardial performance in both groups following propranolol treatment as compared to oxprenolol. This tendency was more marked on the third and fourth measurements, during which higher blood levels are expected, and was statistically significant on measurement IV (p < 0.001, t = 3.5 for the ratios PEP/LVET). A slight tendency for lower values of MVO<sub>2</sub> following the treatment with propranolol as compared to oxprenolol was obtained (measurement I: t = 2.04, p < 0.05; measurement IV: t = 1.56, p < 0.05). It can be concluded that treating hypertensive patients with propranolol is accompanied by a somewhat lower myocardial function and a tendency for lower MVO<sub>2</sub> as compared to treatment with oxprenolol. Treatment with oxprenolol seems, therefore, to be superior to propranolol in patients with impaired myocardial function.