We have studied, in 14 patients undergoing elective aorto-coronary bypass surgery, the effect of the type of ventilation on pulmonary retention of alfentanil and sufentanil using a double indicator technique. Patients were allocated to one of two groups to receive either bolus doses of sufentanil 33.2 micrograms (n = 6) or alfentanil 654 micrograms (n = 8), mixed with indocyanine green. In each patient, pulmonary first-pass retention was studied during apnoea (during the 1-min study period), normoventilation and positive end-expiratory pressure (10 mm Hg) ventilation, the order of which was randomized. After sufentanil, mean (95% confidence interval) first-pass pulmonary retention was 50.9 (41.7-60.1)% during apnoea, 50.8 (42.9-58.8)% during normoventilation and 54.4 (43.7-65.0)% during positive end-expiratory pressure ventilation. After alfentanil, first-pass pulmonary retention was 18.7 (5.4-32.0)% during apnoea, 19.9 (8.3-31.5)% during normoventilation and 16.6 (5.6-27.6)% during positive end-expiratory pressure ventilation. First-pass pulmonary retention of alfentanil and sufentanil was not significantly affected by the type of ventilation.