The myocardial effects of a daily oral dose of atenolol were studied by radionuclide multi-triggered ventriculogram in 10 patients (7 men and 3 women) with mild to moderate essential hypertension, aged 29 to 53 years (mean 43) at rest and during exercise. Before and after two months of treatment with 100 mg/day orally of atenolol, the following variables were recorded: systolic blood pressure (SBP), diastolic blood pressure (DBP), heart rate (HR), ejection fraction (EF), peak filling rate (PFR) and peak ejection rate (PER). Beta-blockade treatment caused a significant drop in SBP and DBP, both at rest and during exercise; HR slowed down at rest and during exercise. PFR diminished at rest and during exercise. PER was reduced at rest and during exercise (p less than 0.001). No significant changes in EF were observed after treatment with atenolol. In conclusion, atenolol impaired left ventricular relaxation in spite of the drop in blood pressure.