1. To test the hypothesis that the reduction of left ventricular weight (LVW) observed during angiotensin-converting enzyme (ACE) inhibition in rats is mainly a response to decreased cardiac load, the effect of enalapril on LVW and blood pressure (BP) in normotensive Sprague-Dawley (SD) and in spontaneously hypertensive rats (SHR) was compared. 2. Enalapril decreased BP by 10% in SD rats and 21% in SHR; and LVW by 6% in SD rats and 12% in SHR. A close linear correlation between BP and LVW was observed in SD (r = 0.65, n = 24, P less than 0.001) and SHR (r = 0.92, n = 15, P less than 0.001). 3. The effect of enalapril on LVW, when related to BP, was almost identical in SD and SHR, as the average reduction of LVW (microgram/g of bodyweight) per mmHg was 7.7 +/- 1.9 versus 7.5 +/- 0.9 in SD and SHR respectively. 4. These data constitute additional evidence to support the view that, at least under ACE inhibition, reduction in LVW is mainly due to the fall in BP and the decreased cardiac load.