In a randomized double-blind trial a total of 50 consenting patients scheduled for elective surgical operations were given multiple intravenous doses of butorphanol tartrate or morphine sulphate in combination with other agents to evaluate and compare the efficacy of these drugs in balanced anaesthesia. Equipotent doses of butorphanol tartrate (mean dose 2.0 mg) or morphine sulphate (10 mg) and thiopentone were employed as induction agents followed by the standardized use of muscle relaxants to facilitate tracheal intubation. Butorphanol tartrate or morphine sulphate were then employed during maintenance of anaesthesia in repeated intravenous doses, averaging butorphanol 4.6 mg and morphine 22.8 mg per patient. Evaluation of anaesthesia showed that induction and course were smooth in 96 per cent of the patients receiving butorphanol tartrate and in 84 per cent of patients receiving morphine sulphate. The analgesic action of butorphanol appeared in every respect to approximate that of morphine sulphate, with negligible side-effects. The data demonstrate that butorphanol is a useful analgesic for use in a balanced anaesthesia technique with a low side-effect incidence.