Neuropharmacological properties of tolperisone hydrochloride (2,4'-dimethyl-3-piperidinopropiophenone hydrochloride) were investigated in mice, rats and cats. Tolperisone inhibited the spontaneous movement and methamphetamine-induced hyperactivity in mice and the ED50 was approx. 50 mg/kg, s.c. At this dose, tolperisone did not prolong the pentobarbital-induced sleeping time. Tolperisone inhibited convulsions induced by pentylenetetrazol, nicotine and maximum electric shock, but did not affect convulsions induced by strychnine and picrotoxin. Tolperisone induced muscle relaxation in mice and rats in several pharmacological tests, but did not affect neuro-muscular transmission. Tolperisone did not affect conditioned avoidance response in rats and methamphetamine-induced rotational behaviour in nigro-lesioned rats. Tolperisone reduced decerebrated rigidity in cats with i.v. administration of 5 approximately 10 mg/kg and intraduodenal administration of 50 approximately 100 mg/kg. Tolperisone elicited a slight drowsy pattern in the spontaneous EEG of cats at 5 approximately 10 mg/kg, i.v., and inhibited the EEG arousal response and pressor response to stimulation of mesencephalic reticular formation or posterior hypothalamic area. These results suggest that inhibition of the activity in the gamma pathway descending from the mesencephalic reticular formation may be involved in the mechanism of muscle relaxant action of tolperisone.