The N-methyl-d-aspartate receptor (NMDAR) antagonist (R,S)-ketamine has robust antidepressant effects in depressed patients although it has detrimental side effects such as psychotomimetic and dissociative symptoms. (R,S)-Ketamine is known to cause the expression of heat shock protein HSP-70 (a marker for neuronal injury) in the retrosplenial cortex of rat brain, suggesting that the neuropathological changes may play a role in the detrimental side effects of (R,S)-ketamine. This study was undertaken to examine whether (R,S)-ketamine and its two enantiomers, (R)-ketamine and (S)-ketamine, causes the expression of HSP-70 in the rat retrosplenial cortex after a single administration. The HSP-70 immunohistochemistry in the rat brain was performed 24 h after intraperitoneal administration of saline (1 ml/kg), (+)-MK-801 (or dizocilpine: 1.0 mg/kg), (R,S)-ketamine (100 mg/kg), (S)-ketamine (25, 50, or 75, mg/kg), or (R)-ketamine (25, 50, or 75 mg/kg). Marked expression of HSP-70 immunoreactivity in the retrosplenial cortex was detected after administration of dizocilpine or (R,S)-ketamine (100 mg/kg). Higher does (50 and 75 mg/kg) of (S)-ketamine, but not low dose (25 mg/kg), caused expression of HSP-70 in this region. In contrast, all doses of (R)-ketamine did not induce the expression of HSP-70 in this region. These findings suggest that marked expression of HSP-70 in the retrosplenial cortex after a single dose of (R,S)-ketamine or (S)-ketamine may have detrimental side effects in the rat brain. Therefore, it is likely that (R)-ketamine is a safer compound in humans than (R,S)-ketamine and (S)-ketamine.