Sixteen patients who had essential hypertension were stabilized on either captopril8 or enalapril monotherapy and had 24-hour blood pressure profiles monitored by using one of two automatic, non-invasive ambulatory systems: Spacelabs 5300 (Squibb, Princeton, NJ) or PAR Physioport II (Kardiotec, Mannheim). In four subjects (group 1), ambulatory pressures were repeated 4 to 6 weeks later using the same equipment and the same drug. In four subjects (group 2), the drug was changed (dose ratio: captopril:enalapril, 5:1) after the first measurement, but the monitoring equipment was not changed. In four subjects (group 3), the drug was constant, but the equipment was changed for the second measurement of ambulatory pressure. In four subjects (group 4), both drug and equipment were reversed after the first measurement. The results showed that both drugs (given once daily) controlled blood pressure during the 24-hour period, with no clinically significant difference between them.