Abstract:Perfusion of isolated rabbit lungs with heparinized whole blood was complicated by the development of a very marked increase in pulmonary vascular resistance (PVR), which occurred in the course of from 10 min to 2 hr. This increase in PVR occurred also when nearly all thrombocytes had been removed from the perfusate. Inhibition of the vasoconstrictor action of noradrenaline, 5‐hydroxytryptamine, histamine or acetylcholine did not prevent the development of the increase in PVR. Plasma kinins, which cause marked vasoconstriction in this preparation, were not present in significant amounts in the perfusate during periods of high PVR. The increase in PVR could be efficiently counteracted by tri‐cresol, 4–8 μ/ml perfusate. Two of the cresol isomers, namely o‐cresol and p‐cresol, were more efficient than the third isomer, m‐cresol. Adeno‐ sine diphosphate (ADP) and adenosine triphosphate (ATP) in doses above a certain level caused vasoconstriction in the preparation. Their vasoconstrictor effect, which increased very markedly during the first part of the perfusion, was inhibited by tri‐cresol. Also the vasoconstrictor effect of noradrenaline was reduced by tri‐cresol. Apart from these actions and from that on the spontaneously increasing PVR, tri‐cresol had no other apparent effects on the preparation. A relationship between the increase in PVR and effects of ATP and ADP in the preparation seems likely.