The pharmacological properties of 4''-ethyl-2-methyl-3-pyrrolidinopropiophenone hydrochloride (HSR-770), a new centrally acting muscle relaxant, were investigated in experimental animals. 1. HSR-770, within a dose range of 1-10 mg/kg i.v., relaxed .alpha.- and .gamma.-rigidities in rats, the dose required to relax .gamma.-rigidity being lower than the dose required to relax .alpha.-rigidity. When HSR-770 was intraduodenally (i.d.) administered within a dose range of 25-50 mg/kg, the relaxant activity on .alpha.-rigidity was potent and long-lasting in comparison with eperisone or tolperisone. 2. HSR-770 inhibited flexor reflex more strongly than patellar reflex in a dose-dependent manner within a dose range of 2.5-10 mg/kg i.v. or 25 and 50 mg/kg i.d. in anesthetized cats. In spinal cats, HSR-770 (5 and 10 mg/kg i.v.) inhibited flexor reflex but this potency was weaker than that in anesthetized cats. 3. HSR-770 (12.5-50 mg/kg i.d.) inhibited the crossed extensor reflex in anesthetized rats. 4. In spinal cats, HSR-770 inhibited mono- and polysynaptic reflex potentials to the same extent and also depressed dorsal root reflex potential at 5 and 10 mg/kg i.v. 5. HSR-770 at a dose (50 mg/kg p.o.) which exerted muscle relaxant activity on both rigidity and flexor reflex, had litte effect on spontaneous motor activity (mice), hexobarbital-induced sleeping time (mice) and conditioned avoidance response (rats). These resutls indicate that HSR-770 is a potent centrally acting muscle relaxant and that its central nervous system depressant activity is relatively weak.