The performance of the Ohio 5400 electronic anemometer and the Ohmeda SE302 heated platinum filament respirometer were assessed in a series of bench tests in which they were incorporated into the patient circuit of a Siemens-Elema Servo 900C ventilator. The reference instrument was a Fleisch pneumotachograph. The Ohio 5400 was accurate to within 10% when measuring air flows between 23 and 44 l/min, provided the cycling frequency did not exceed 30 b.p.m. and the flow wave form was not accelerating. The Ohmeda SE302 was accurate to within 10% when measuring air flows between 8 and 48 l/min, regardless of cycling frequency or shape of respiratory wave form. Both instruments displayed marked inaccuracies (of the order of 20-30%) when measuring 100% oxygen or 66% nitrous oxide/oxygen mixtures. The performance of the flow transducer of the Servo 900C was also assessed. This was significantly superior, and its output was virtually identical to the uncorrected output of the pneumotachograph.