A single work rate paced step test is described. It was designed to allow the measurement of minute ventilation (VI) and oxygen consumption (VO2), under standardised conditions, during exercise in the clinic and lung function laboratory and in field work. The subjects and the operator found the test simple to perform. The values for ventilation at a given oxygen consumption were similar to those from more complex and stressful tests and had a high degree of reproducibility. The ergometer was a 12 inch (30 cm) step with hand rails. The subjects stepped down, once every four seconds in response to a buzzer for 10 minutes. VI and VO2 were measured with a small portable device. In 53 normal subjects of mean weight 69 (range 49-107) kg, mean VO2 measured during the last 5 minutes of the test was 0.89 (range 0.53-1.52) 1/min. Weight and height were independent contributors to the oxygen cost of stepping. The ventilatory equivalent for oxygen (VIO2, which equals VI divided by VO2) increased with age, and was similar to normal values for ventilation at 1.01/min VO2 obtained by workers who used multiple work rate tests on more complex equipment. The within subject variation in VIO2, expressed as the coefficient of variation (cv) and calculated for each age decade, was 14%. This again compared very favourably with the cv for values of ventilation at 1.01/min VO2 from other studies. In 20 normal subjects daily testing showed significant falls in VO2 and V1 over three days of 4% and 5.9% respectively; VIO2 fell by 2%. Testing over four consecutive weeks showed small significant differences between weeks but no clear trend; the within subject weekly variation accounted for less than 3.5% of the total variance for each of the variables. Testing at least four months apart showed a 3.3% fall in the oxygen cost of stepping. No significant diurnal effects on the exercise responses were found.