To evaluate the effect of converting enzyme inhibition induced by enalapril on parasympathetic activity, we studied ten essential hypertensive patients, age range 38-58 years, WHO I-II. Parasympathetic evaluation was obtained by measuring the variation of heart period (VHP) during at least 1 minute of steady-state, regular respiration. VHP was derived from the difference between the mean of all maximum and the mean of all minimum heart periods. The higher the VHP, the higher the parasympathetic control of heart rate and vice versa. VHP was measured supine and with tilting (30 degrees, 60 degrees, 85 degrees). Blood pressure was reduced after 1 month of enalapril treatment, while the heart rate did not change. VHP increased at the end of enalapril treatment compared with placebo: in the supine position it increased from 36 +/- 3.2 ms to 44 +/- 3.5 ms, p less than 0.01. VHP was also increased by enalapril at 30 degrees (p less than 0.05) and 60 degrees (p less than 0.05), while no difference was observed at 85 degrees between placebo and enalapril. A positive correlation was found between supine enalapril changes of VHP and those of systolic and diastolic BP. In conclusion, enalapril seems to increase parasympathetic cardiovascular control in essential hypertensive patients. This result might explain the lack of increase in heart rate that would be expected as a result of the vasodilating effect of enalapril.