The pulmonary kinetics of fentanyl and alfentanil were studied quantitatively in two groups of five and six surgical patients, respectively, after the induction of anaesthesia. A mixture of one of the opioids and indocyanine green was administered as a bolus. From measurements of the concentrations of the dye in plasma and the opioids in blood, central blood volume and the amount of drug which passed through the pulmonary circulation were calculated. The pulmonary release of fentanyl and alfentanil was calculated from the arterial-mixed venous blood concentration differences. Fentanyl 43.0-86.9% (median 70.9%) and 35.9-79.8% of alfentanil (median 58.6%) were sequestered by the lung on first passage of the opioid-containing blood through the pulmonary capillaries. During the following 14 min, fentanyl was released, apparently from two binding sites (T1/2fast: 0.22 (0.16-0.27) min; T1/2slow: 5.78 (3.65-13.86) min). Initially, there was a rapid release of alfentanil (T1/2: 0.28 (0.08-0.51) min). However, 1-3 min after injection, the arterial-mixed venous blood concentration differences of alfentanil disappeared, although considerable amounts of drug were still present in the lung of five of the six patients studied. It may be expected that a temporary pulmonary sequestration of fentanyl and alfentanil has considerable impact on the time course of their pharmacological effects, on the time necessary to reach their maximum effect and on the intensity and (in case of fentanyl) the duration of these effects.