The azeotropic mixture of halothane and diethyl ether (HE) may be a valuable alternative to other anaesthetic agents under circumstances such as war, civil disaster, and primitive conditions. In the present study the minimum alveolar concentrations (MACs) for HE in man (n = 25) and in pigs (n = 6) were determined. For comparison with results of other studies, the MAC for halothane in man (n = 14) was also determined. A Normac gas analyser and a Varian 3700 gas chromatograph were calibrated against known standards of HE. The performance of two vaporizers (Fluotec Mark III, Servo vaporizer for halothane) was studied. The Fluotec Mark III vaporizer and the Servo halothane vaporizer filled with HE gave a linear performance with increasing dial settings. The Normac gas analyser set for methoxyflurane was linearly sensitive to the ether component of the azeotrope. The MAC for HE in man was 0.71 vol.% 0.03 (s.e.mean) (ether 0.21 vol.%, halothane 0.50 vol.%) in the age group 19–42 years. MAC for halothane in man was 0.65 0.03 in the age group 19–32 years. The MAC for HE in pigs between 20–24 kg was 0.99 vol.% 0.07. The ether component seems to act synergistically with halothane in the azeotropic mixture. The present study provides an opportunity to compare HE with other volatile anaesthetic agents.