Sixteen hypertensive male out-patients (33-54 y), whose blood pressure (BP) had been normalized (diastolic BP < 90 mmHg) by treatment with a daily dose of 50 mg atenolol (CAS 29122-68-7), participated in this double-blind, placebo-controlled, parallel group study, which investigated the possible influence of the non-steroidal anti-inflammatory drug tenoxicam (CAS 59804-37-4) on the control of BP by atenolol. After a run-in of 10 days, to assess the stability of BP control by atenolol, and to determine baseline parameters, 8 patients in group A received 20 mg tenoxicam (2 x 20 mg on days 1 and 2), and 8 patients in group B received placebo, daily over 15 days (days 0-14), concomitantly with their atenolol regimen. BP was measured under standardized conditions on several days. Heart rate (EHR) after 5 min of exercise by bicycle ergometry (constant 75W), and parameters of renal function were assessed before (baseline) and during concomitant dosing of atenolol and tenoxicam. On day 14 the mean changes (delta A, delta B) from baseline of pre-dose BP (mmHg) and EHR (beats/min) in groups A and B, and the one-sided 95% confidence regions (R) for delta A, respectively, were (delta A, delta B, R): 4.4, 1.6, < 9.5 for sitting systolic BP, 2.8, -0.3, < 4.5 for sitting diastolic BP, -0.3, -0.6, < 5.5 for standing systolic BP, -0.6, -1.9, < 3.0 for standing diastolic BP, 0.4, -7.5, < 0.4 for EHR at pre-dose, 3.1, 0.6, < 7.8 for EHR at 3.5 h post-dose.(ABSTRACT TRUNCATED AT 250 WORDS)