Systemic salt loading has been reported to facilitate operant heat-escape/cold-seeking behavior. In the present study, we hypothesized that the median preoptic nucleus (MnPO) would be involved in this mechanism. Rats were divided into two groups (n = 6 each): one group had the MnPO lesion with ibotenic acid (4.0 µg) and the other was the vehicle control. After s.c. injection (10 ml kg^-1) of either isotonic- (154 mM) or hypertonic-saline (2500 mM), each rat was placed in a behavior box, where the ambient temperature was changed to 26°C, 35°C, and 40°C every 1 h. The position of a rat in the box and the body core temperature (T_core) were monitored. A rat could trigger 0°C air for 45 s in the 35°C and 40°C heat when moved in a specific area in the box (operant behavior). In the control group, counts of the operant behavior were greater (P < 0.05) in the hypertonic- than in the isotonic-saline injection (17 ± 2 and 10 ± 2 at 35°C, 24 ± 2 and 18 ± 1 at 40°C). T_core remained unchanged throughout the exposure, although the level was lower (P < 0.05) in the hypertonic- than in the isotonic-saline trial (36.6 ± 0.2°C and 37.4 ± 0.1°C at 26°C and 36.9 ± 0.2°C and 37.4 ± 0.1°C at 40°C, respectively). However, in the MnPO-lesion group, counts of the behavior were similar between the hypertonic- and isotonic-saline injection trials (10 ± 2 and 8 ± 1 at 35°C, and 17 ± 1 and 16 ± 1 at 40°C, respectively). T_core increased (P < 0.05) in the heat in both trials (36.8 ± 0.1°C and 37.4 ± 0.1°C at 26°C and 37.4 ± 0.2°C and 37.8 ± 0.2°C at 40°C in the hypertonicand isotonic-saline injection trial, respectively). These results may suggest that, at least in part, the MnPO is involved in the facilitation of heat-escape/cold-seeking behavior during osmotic stimulation.