The effects of fentanyl on total respiratory pressure-volume relationship were studied in 12 supine, anesthetized (thiopental, succinylcholine, halothane, N2O/O2) children, 5, 10 and 15 min after administration of fentanyl, 3 micrograms/kg (Group F) and at similar intervals in 11 control, anesthetized children who did not receive fentanyl (Group C). Lung and chest wall pressure-volume relationships were measured in five children from Group F and in six from Group C. Compliance values were estimated from the linear part of PVR curves. In Group F, a significant decrease was observed at 5, 10 and 15 min (maximum 29%) for total compliance and at 15 min (maximum 26%) for lung compliance. Chest wall compliance was reduced (maximum 45%) at 15 min, but not significantly. In Group C, no change in compliances was noted. The comparison between the two groups for total, chest wall and lung compliances showed a significant decrease in these three parameters at 15 min in children receiving fentanyl. The decrease of chest wall compliance can be explained by increased muscle tone, but the mechanism of decreased lung compliance awaits further study.