Colloid osmotic pressure (COP) during hypothermia with and without total circulatory arrest was evaluated in 20 adult mongrel dogs (average body weight = 18.6 kg). All animals were surface-cooled to 20 degrees C (rectal temperature) under halothane-diethyl ether azeotrope anesthesia and divided into 4 equal groups. In Groups I (non-arrest) and II (60 minute-arrest) 10% low molecular weight dextran (LMWD; 1 gm/kg = 10 ml/kg) was administered during cooling. Physiological saline (10 ml/kg) was given in Groups III (non-arrest) and IV (60 minute-arrest) during cooling. COP in LMWD groups increased significantly during cooling and was consistently higher than it was in the saline groups during rewarming. There were no significant differences in heart rate (HR), mean arterial pressure (MAP), central venous (CVP) and pulmonary artery wedge pressures (PAW) between the 4 groups during cooling. The increase in hematocrit (Hct) in animals that received saline was significant at the end of cooling and during rewarming. Hct was not significantly changed in Group I throughout the procedure, whereas in Group II (60 minute-arrest) it increased significantly during rewarming despite the administration of LMWD. Rewarming time in Group II was significantly shorter than in Group IV (144.2 +/- 9.6 min vs 193.2 +/- 32.6 min, respectively). We conclude that the administration of LMWD effectively reduces hemoconcentration and is also beneficial for maintaining better peripheral circulation.