Calcium entry blockers are now widely employed in the treatment of cardiovascular diseases and perioperative hypertension. In patients with coronary heart disease nifedipine therapy should be continued perioperatively to avoid coronary artery spasm. Animal experiments have demonstrated that calcium entry blockers potentiate the neuromuscular blockade induced by nondepolarizing blocking agents. In patients, an atracurium-induced neuromuscular depression is prolonged by intravenous nifedipine. In this prospective clinical study we evaluated the effect of chronic oral nifedipine therapy on the duration of neuromuscular block by atracurium. Sixty patients anaesthetized with isoflurane in nitrous oxide/oxygen were recruited for this study. Thirty of these were on chronic oral nifedipine therapy and received their normal morning dose before premedication. The control consisted of 30 patients of similar age and status but not taking any calcium entry blockers. Monitoring included noninvasive blood pressure, heart rate, pharyngeal temperature, physical breathing parameters and neuromuscular transmission with a Datex Relaxograph TM ("train of four"-principle). After inducing hypnosis 0.5 mg/kg atracurium were administered for muscular relaxation. The duration of block from administration of the relaxant to recovery of first twitch height (T1) to 25% of control twitch height was registered as duration of initial block. When T1 reached 25% a repetition dose of 0.2 mg/kg atracurium was injected. The time till recovery of T1 to 25% was recorded as the duration of the repetition dose. Results were compared using Student's t-test for unpaired data. There was a significant prolongation of the duration of initial block from 38 min +/- 10 min in the control group to 46 min +/- 8 min in the therapy group (P < 0.01). The duration of the repetition dose rose from 30 min +/- 8 min in the control group to 38 min +/- 7 min in the therapy group (P < 0.001). Daily nifedipine doses varied from 10 mg in the morning to 40 mg divided into single doses with no influence on the prolongation of neuromuscular block. Our results confirm previous assumptions of synergistic effects of nifedipine and neuromuscular blocking drugs in patients. Chronic oral nifedipine therapy potentiates neuromuscular blockade by atracurium as does nifedipine intravenously. This effect should be considered in the treatment of cardiovascular diseases with nifedipine in the perioperative period.