Atracurium, a new non-depolarizing neuromuscular blocking agent, was studied in 70 patients anesthetized with fentanyl, thiopental, and nitrous oxide-oxygen. The dose found to produce 95% twitch inhibition (ED95) was 0.2 mg/kg. The onset time from injection to maximum depression of twitch was 4.0 minutes at this dose; the duration to 95% recovery was 44.1 minutes. Twice the ED95 dose (0.4 mg/kg) had an onset time of 1.7 minutes and a duration of 63.5 minutes. No cardiovascular effects were observed in this dosage range. At higher doses (0.5 and 0.6 mg/kg) arterial pressure decreased 13% and 20% and heart rate increased 5% and 8%, respectively. Sixteen patients received at least four successive doses of atracurium. No clinically significant cumulative effect could be shown when recovery from 25% to 75% of control twitch height was compared for initial and final doses in the series. Atracurium spontaneously decomposes at physiologic pH via the Hofmann elimination reaction and may also undergo ester hydrolysis independent of plasma cholinesterase. These proposed pathways of inactivation may explain the lack of cumulative effect and the drug's intermediate duration of action. Based on the results of this study, atracurium offers several clinical advantages and should undergo more extensive clinical trials.