Effects of some diphenyl and monophenyl compounds on grip strength in mice and spinal reflexes in young chicks were investigated in order to study structure-activity relationships between muscle relaxant activity and neolignane compounds, magnolol and honokiol extracted from Magnolia officinalis THUNB. Diphenyl produced a long-lasting suppression in the spinal reflex and relatively weak inhibition in the grip strength. An introduction of a hydroxyl into 2-position of diphenyl, o-phenylphenol, increased the muscle relaxant activity and accelerated the onset, although the duration was still long. In the spinal reflex preparation the duration of action became short. The introduction of two hydroxyls into 2- and 2'-position of diphenyl, 2,2'-dihydroxydiphenyl, further strengthened the activity and shortened the duration of the inhibitory effect on the grip strength and the spinal reflex. When two allyls are introduced into 5,5'-position of 2,2'-dihydroxydiphenyl, it corresponds to magnolol. Magnolol, 5,5'-diallyl-2,2'-dihydroxydiphenyl, produced potent inhibitory effects of gradual onset and of long duration on the two test preparations. Position of allyls and hydroxyls in honokiol, 5,3'-diallyl-2,4'-dihydroxydiphenyl, is different from magnolol, although the pharmacological characteristics are quite similar to magnolol. These results suggest that a hydroxyl accelerates the onset and shortens the muscle relaxant activity of diphenyl and an allyl influences the activity in the opposite direction. Both radicals appear to intensify the activity.