1. A randomised, placebo controlled, double-blind, parallel group study was conducted to assess the effect of tenidap sodium, a novel cytokine modulating drug, on the stable hypotensive response to the angiotension converting enzyme (ACE) inhibitor enalapril in subjects with mild to moderate, uncomplicated, essential hypertension. 2. Twenty-four male and female hypertensives, aged 33-77 years, received either 120 mg tenidap sodium or matched placebo daily for 22 days concomitantly with enalapril. 3. Mean endpoint supine and standing, systolic and diastolic pressures remained within 10% of baseline in each treatment group. However, the endpoint values were marginally above baseline during double-blind treatment with tenidap and marginally below baseline in the group receiving placebo. The increases in supine and standing systolic pressures in the tenidap group differed significantly from the changes in the placebo group. There were no significant differences between groups in changes in pulse rate. 4. Gastrointestinal side effects of mild to moderate severity attributed to treatment with tenidap were experienced by five subjects, one of whom was withdrawn during the third week of treatment. One subject receiving placebo was withdrawn because of a moderate headache attributed to study treatment. 5. The results of this study suggest that treatment with tenidap may interfere with the anti-hypertensive efficacy of ACE inhibitors. It is recommended that blood pressure should be monitored when tenidap is administered concomitantly with an ACE inhibitor.