Althesin was compared with thiopentone in the induction of anaesthesia in 240 patients who received Althesin 30, 50 or 70 μ1/kg or thiopentone 3, 5 or 7 mg/kg. Half of the patients received pethidine and atropine for premedication and 120 promethazine in addition. According to the ED50 values for the loss of eyelid reflex Althesin 30 μ1/kg was roughly equipotent to thiopentone 3 mg/kg. The dose-response curve of thiopentone, however, seemed to be steeper than that of Althesin. The ratio ED50 apnoea/ED50 loss of eyelid reflex was higher for Althesin (0.86) than for thiopentone (0.69). The changes in blood pressure and heart rate both during induction and operation were similar but the incidence of per- and postoperative untoward effects was slightly though not significantly higher after Althesin.