1. Twelve untreated hypertensive patients whose blood pressure was 171.8 ± 5.5 mmHg systolic and 119.7 ± 3.4 mmHg diastolic (mean ± sem) were treated aggressively with diuretics plus other antihypertensive agents. Echocardiograms were performed before, and 2 weeks, 3 months and 6 months after therapy. Blood pressures were lowered to an average of 142/98 mmHg over the 6 month period. 2. Mean velocity of circumferential fibre shortening rose from 1.1 ± 0.09 to 1.3 ± 0.06 diameters/s at 2 weeks and remained elevated at the end of 3 months (1.3 ± 0.03 diameters/s) ( P <0.025), but returned to the control level in 6 months. Similarly, ejection fraction increased significantly during the same period from a control value of 65.1 ± 4.4 to 73.4 ± 1.8% ( P <0.025) and persisted in this range at 3 months. At 6 months the ejection fraction had returned to pretreatment levels. There were significant reductions in left ventricular end-systolic and end-diastolic dimensions. Left ventricular mass index decreased from 182.3 ± 18.3 to 163.8 ± 12.4 g/m 2 after 6 months of therapy. 3. These results indicate that in the early stages of blood pressure reduction there is a temporary increase in ejection phase indices, probably related to afterload reduction. The reduction in the left ventricular mass index suggests that increased cardiac muscle mass due to elevated blood pressure may be partially reversible after long-term reduction in blood pressure.