OBJECTIVE: We have undertaken a laboratory study to determine whether a drawover vaporizer in the inspiratory limb of an intensive care ventilator circuit can produce safe and therapeutic concentrations of isoflurane. DESIGN: An Oxford Miniature Vaporizer (OMV) and Ohmeda TEC vaporizer were assessed over the range of inspired isoflurane concentrations, airway pressures and tidal volumes experienced in the ICU. CONCLUSIONS: The experimental findings suggest that the OMV inhaler in plenum mode can be relied upon to produce safe concentrations of isoflurane over a clinically useful range of inspired concentrations. Furthermore, it behaves predictably over the range of airway pressures likely to be encountered in the patient admitted with acute severe asthma. However, we found that the Ohmeda TEC vaporizer did not perform reliably in this setting.