An electronic respirometer is described. Because of the calibration procedure at a frequency of 20/min the instrument tends to give a small underestimate at frequencies of less than 20/min and an overestimate at frequencies greater than 20/min. If the instrument is used to measure respirable gas mixtures the accuracy is well within the limits claimed (±2%, ±1%, f.s.d with a variable flow) provided allowance is made for such artefactual errors as flow reversal. A greater accuracy was obtained with constant flow, provided allowance was made for a constant under-reading 1.45. l./min.