A comparative study was undertaken to investigate the neurotoxicity of ornidazole enantiomers administered i.v. in Beagle dogs. Beagle dogs were administrated i.v. S-ornidazole, R-ornidazole, rac-ornidazole and vehicle (0.9% sodium chloride injection containing 5% propylene glycol) for 2 wk, resp. General status and body weight were observed once time every day during the treatment period. At the end of the treatment, gross anatomy and histol. evaluation were carried out. The following symptoms were observed after 1-3h given S-ornidazole (200mg/kg, iv): salivation, vomit, urinary and fecal incontinence. Gain in body weight and food consumption was lightly depressed, which were not different with the control group. Salivation, vomit, myasthenia of limbs, inability to stand and twitch was appeared when given R-ornidazole at the same dose level as S-ornidazole. These symptoms were gradually severity during the course of the administration period. Meanwhile recovery time was prolonged. Gain in body weight and food consumption was obviously depressed, which were significantly different with the control group. The toxic effect of rac-ornidazole was moderate. Histol. examination showed that Purkinje cell in the R-ornidazole group had lightly degeneration, and the numbers were lower than those in S-ornidazole groups. There was no structural abnormality in other examined organs. The toxic effects of S-ornidazole are much slighter than that of R-ornidazole, and cerebellum in R-ornidazole group has shown Purkinje cell degeneration and numbers decreased under the present study conditions. These results suggest that S-ornidazole has light toxic effects, which may result in much safer in clin. usage, especially decrease incidence rate of side effects in central nervous system.