N-Substituted 3-amino-2-methyl-N-(3-phenyl-5-isoxazolyl)propanamide derivatives (5a-w) were prepared, and their muscle relaxant and anticonvulsant activities were evaluatd. Among the compounds with high potency, 3-diethylamino-2, N-dimethyl-N-(3-phenyl-5-isoxazolyl)propanamide (5e) was found to possess selective muscle relaxant and anticonvulsant activities. Compounds with carbon side chains other than the 2-methylpropanamide moiety did not give increased muscle relaxant activity. Optical isomers of 5e were prepared from metyl (+)-and (-)-3-diethylamino-2-methylpropionates (15a, b), and the (+)-isomer (5x)b was found to be twice as potent as the (-)-isomer (5y). The structure-activity relationship was studied with emphasis on the effects of the 3-amino moiety and of the substituent on the isoxazolyl 5-amino group. A good quadratic correlation equation was found between hydrophobicity and muscle relaxant activity.