We have studied the pulmonary extraction and retention of sufentanil, alfentanil and morphine using a double indicator technique in 30 patients undergoing elective aortocoronary bypass surgery. Patients were allocated to three groups (10 each) to receive sufentanil 43 micrograms, alfentanil 672 micrograms or morphine 1887 micrograms, mixed with indocyanine green as indicator. After sufentanil, mean peak extraction was 93.7% (95% confidence interval 87.1-100.4%) and release occurred after 16.1 (13.8-18.4) s; first-pass retention was 61.1 (51.3-70.9)%. After alfentanil, peak extraction was 67.4 (42.7-92.0)% and release occurred after 9.9 (8.5-11.3) s; first-pass retention was 10.1 (3.5-16.7)%. After morphine, peak extraction was 58.3 (44.4-72.2)% and release occurred after 7.2 (4.4-10.1) s; first-pass retention was 7.1 (-4.7-19.9)%. Both peak extraction and first-pass retention were significantly greater after sufentanil. There was no significant difference in the peak extraction and first-pass retention between alfentanil and morphine.