A study was made of the effect of unilateral injection of taurine into the substantia nigra on the behaviour of rats. Taurine (10-200 micrograms) induced a dose-dependent contralateral circling behaviour. The maximum intensity of circling after doses of 100-200 micrograms of taurine was reached within 10 min. and the circling lasted for about 4-5 hrs. There was neither ipsilateral circling nor stereotyped behaviour. Intranigral injection of isethionic acid, a metabolite of taurine, induced weak ipsilateral circling of short duration. Pretreatment with bicuculline (3 mg/kg intraperitoneally) or strychnine (0.25 mg/kg intraperitoneally) significantly inhibited the taurine-induced circling. A dose of 1 mg/kg but not of 0.5 mg/kg of haloperidol (subcutaneously) significantly decreased the intensity of the taurine-induced circling. Pretreatment with atropine (10 mg/kg intraperitoneally) had no significant effect on the circling behaviour. Besides the dopaminergic nigrostriatal pathway, the nondopaminergic nigral output pathways also seem to be involved in the taurine-induced circling behaviour. The results show that taurine may play some role in the function of the substantia nigra.