Pulmonary mean-transit-time volume (PBV) of 12 anesthetized dogs was measured repeatedly by using pairs of dye-dilution curves recorded from the pulmonary artery (PA) and left atrium (LA) after right atrial injections of indocyanine green. In 345 pairs, the areas of the extrapolated LA curves exceeded those of the PA curves by 8% (sd, 14%), indicating that delayed color development of the dye-blood mixture is not a serious source of error in standard dye-dilution techniques. In eight dogs, measurement every 3–5 min before and after 2- or 5-min infusions of morphine (1 mg/kg) revealed no change in PBV, although arterial pressure decreased and cardiac output increased transiently. Prolonged (40 min) intra-arterial infusions of ATP, acetylcholine, or norepinephrine into four other dogs caused large changes in systemic and intracardiac pressures and in heart rate but not in PBV.