In human volunteers, Forane failed to alter twitch height, but increased average neuromuscular refractory period and prevented a sustained response to stimulation at higher frequencies of tetanus. These effects were dose-related. Surgical patients anesthetized with 1.25 MAC Forane were unable to sustain tetanus at more than 120 Hz. In contrast, patients anesthetized with 1.25 MAC halothane were able to sustain tetanus at 200 Hz. In 15 patients anesthetized with 1.25 MAC Forane, the median effective dose ofd-tubocurarine needed to produce a 50 per cent depression of twitch height (ED50) was 1.70 mg/m2. In contrast, the ED50ofd-tubocurarine in 17 patients anesthetized with 1.25 MAC halothane was 5.69 mg/m2. Thus, 3.3 times as muchd-tubocurarine was needed to produce a 50 per cent depression of twitch height with halothane.