The anaesthetic properties of pregnanolone emulsion, which is a new pharmaceutically and clinically acceptable preparation of pregnanolone, have been evaluated and compared with those of thiopentone in the mouse. "Loss of the righting reflex" for 15 sec. was used as a criterion for the anaesthetic effect. The range for the optimal infusion rate to reach the threshold anaesthetic dose was 0.25-1.0 mg/kg/min. for pregnanolone and 10-40 mg/kg/min. for thiopentone. Potency, measured at the optimal infusion rate as the mean anaesthetic dose, AD50, or as the threshold anaesthetic dose, was about six times higher for pregnanolone than for thiopentone. The induction of anaesthesia, which ensured directly after a complete dose was given, and the duration of anaesthesia were similar for pregnanolone and thiopentone in equipotent doses. The approximate therapeutic index for pregnanolone was about four times higher than that for thiopentone. The range for the time of anaesthesia was about 150 min. for pregnanolone emulsion but only about 50 min. for thiopentone. Recovery of the mice from anaesthesia occurred within minutes after pregnanolone, whereas it took hours following thiopentone. In conclusion, these results show that the advantage of pregnanolone emulsion over thiopentone include a higher therapeutic index, a wider range for the duration of anaesthesia and a shorter recovery period.