The concentratiuon of toluene in the alveolar air was measured in 20 males and 17 females exposed for 7 h either to a constant exposure to 100 ppm toluene or to a varying exposure with the same timeweighted average, but with peaks of 300 ppm every 30 min Both exposure schedules included 50 to 100 W exercise in three 15-min periods Repetitive measurements of the toluene concentrations in the alveolar air were made in two 30-min periods, one at rest and one including work At rest the alveolar concentration increased rapidly in response to an increase in the inspiratory air concentration, while exercise delayed this increase by about 2 min, probably due to an altered distribution of toluene in the body The average alveolar concentration was 16 5 + 6 8 ppm (mean ± SD) at rest and 19 5 ± 5 3 ppm in the period including exercise while there was no difference between constant and varying exposure The alveolar toluene concentration tended to be higher in females than in males both at rest and during exercise Subjects exercising with an intensity of 100 W had 25 % higher values at rest than those exercising at 75 W The excretion of the metabolites hippuric acid and orthocresol in the last 3 h of exposure was correlated to the alveolar toluene concentration at rest but not during work. Besides this, body height and weight influenced the excretion rates, still leaving a large unexplained interindividual variation.