The effects of thiophosphoric acid alkaloid derivatives from Chelidonium majus L. (Ukrain, UKSR-222) on the central nervous system (CNS) of mice and rats was studied. Intraperitoneal (ip) administration of Ukrain in doses of 9.5 and 19 mg/kg for mice depressed spontaneous motor activity, decreased body temperature and potentiated the action of hexobarbital. Only in a dose of 19 mg/kg Ukrain produced analgesic action in the hot plate test. It had no protective effect against electroshock or pentetrazol-induced seizures. In rats, ip administration of Ukrain in dose of 14 and 28 mg/kg potentiated the action of amphetamine and apomorphine but had no effect on catalepsy induced by haloperidol. Ukrain used in dose 9.5, 14, 19 and 28 mg/kg antagonized the head twitches induced by 5-HTP and hyperthermia-induced by m-CPP. Biochemical studies indicated that Ukrain did not affect the NA and DA concentrations in the whole rats' brain and did not affect the 5-HT and 5-HIAA concentrations in the whole brain of rats. These findings demonstrate that the central action of Ukrain involves the stimulation of the dopaminergic system and the inhibition of the serotoninergic system.