Measurements of relaxed expiratory volume in one second (REV1·0) and forced inspiratory volume in one second (FIV1·0) were made on 50 subjects on two occasions to know the variability of these procedures in the individual subject. The mean coefficients of variation for REV1·0 and FIV1·0 were 6·74% and 8·92%, respectively. The 95% confidence limit of difference between highest readings obtained on two occasions for REV1·0 and FIV1·0 was 0·38 litre and 0·214 litre, respectively. These results compare favourably with similar data reported for other ventilatory tests. We feel that if a little time is spent teaching subjects to produce REVs, then reproducible results may be obtained.