The effects of hypothermic perfusion have been studied by using different perfusates in 24 isolated rabbit lung preparations, divided into three groups: G1, perfused with blood (hematocrit of 10%) and G2 and G3, perfused with erythrocyte-free plasma plus 6% protein in saline. In both G1 and G2 groups left atrial pressures were kept below airway pressure (Zone II conditions), and in G3 it was higher than airway pressure (Zone III conditions). Perfusate flow, pulmonary artery pressure, pulmonary vascular resistance, left atrial pressure, fluid filtration rate, colloid osmotic pressure and temperature were not different (p > 0.1) between G1 and G2 at the beginning of the experiments. Lowering perfusate temperature from 38 degrees C to 28 degrees C produced a significant increase in pulmonary artery pressure and pulmonary vascular resistance in G1 but they decreased in G2 lungs (p < 0.05). Fluid filtration rate increased in both groups during hypothermia. These responses were not inhibited by an alpha-adrenergic receptor blocker or a pulmonary vasodilator. In G3 lungs no changes were observed. The differences in the hemodynamic effects of hypothermia observed in G1 and G2, both in Zone II conditions, could result from the differences in the vessel distention state obtained by each of the perfusate before initiating hypothermia. As perfusate viscosity increase with cold, a greater possibility of vessel distention in G2 lungs occurs. This explains the decrease in pulmonary artery pressure and pulmonary vascular resistance with cold in this group. The increase in fluid filtration rate observed with hypothermia in G1 and G2 may be due to increases in fluid exchange area.