The effects of intravenous verapamil (18 micrograms, 36 micrograms, 72 micrograms kg-1) on neuromuscular transmission were studied in the integrated electromyographic (IEMG) responses of indirectly stimulated thenar muscles of five myasthenia gravis patients and 12 other patients without neuromuscular diseases anesthetized with thiopentone, pethidine and nitrous oxide in oxygen. In all subjects, verapamil 72 micrograms kg-1 significantly prolonged the P-R interval of the electrocardiogram from 0.122 +/- 0.004 to 0.152 +/- 0.005 sec and decreased the heart rate from 78.7 +/- 9.5 to 69.6 +/- 6.6 beat min-1, but it had no effect on arterial blood pressure. Verapamil 36 micrograms, 72 micrograms kg-1 significantly inhibited the IEMG to 84.50 +/- 3.30% and 79.25 +/- 3.52% of control value, respectively, in the myasthenia gravis patients. However, these doses did not influence the IEMG of normal subjects. The specific site and mechanism of verapamil's neuromuscular blocking action remain unclear. It is concluded that verapamil can produce neuromuscular blocking action in myasthenia gravis patients and that therefore caution is needed in administering verapamil to such patients.