TIlE clinical advantages of supplementing N~O-O2 a,naesth~ia with Pentothal| Sodiumt and an analgesic of the morphine class, rather than with Pentotbal alone, have been adequately demonstrated (1, 2, 3, 4, 5, 6). It has been shown recently by Siker et al. (7) that Nisentil Hydrochloride~ (1, 8-dimethyl-4-phenyl-4-propionoxy-piperidine hydrochloride), because of :its short duration of action and excellent controllability, is the analgesic of choice for this purpose. These workers reported that the mg/min dose of Pentothal was glreatly decreased ff Nisentil rather than Demerol,@ was used for supp]ementation. However, their attempts to reduce further the Pentothal requirements, by hacreasing the Nisentil dose, fated because of the marked re~r~iratorv deraression encountered. Since studies by Swerdlow et al.-(8-) in unanaesthetized patients showed that the narcotic antagonist, levo-8-hydroxy-N-aUylmorphinan tartrate (levallorphan tartrate),~ offers considerable protection against Nisenttl-induced respiratory depression ff these agents are used in a 1:50 ratio, the idea occurred to one of us (L.A.P.) that the eombined use of Nisentil and ]evallorl3han for supplementation of N~O-Oz-Pbntothal anaesthesia may make it possible to use larger quantities of Nisentil, without producing respiratory depression, and thus reduce further, or eliminate completely, the Pentothal requirements. This seemed desirable in view of the relatively slow rate of degradation of Pentothal in the body (9, 10). Consequently, clinical trials which would utilize Nisentillevallorphan combinations were initiated. So far 452 patients, who underwent a variety of surgical procedures, were studied. Of these, 182, who did not require endoxacheal inhabation, did not receive a muscle relaxant; 78 were given Anectine~ (succinylcholine) Chloride for intubation only; and in 192 relaxation was maintained with Anectine throughout anaesthesia. In these three groups 90, 87, and 75 patients, respectively, received NisentiI and levallorphan from a premixed solulion and the remaining 250 patients were given levallorphan first followed by Nisentil. By thus combining these two agents, satisfactory operating conditions could be obtained without the use of Pentothal in 58, or 82 per cent, of the 182 patients who did not