Effects of centrally acting muscle relaxants on the tonic and phasic tension of the forelimbs were studied in the rigid rats. Anemic decerebrate rigidity of rats was obtained by ligation of both basilar and common carotid arteries. Tension recorded from forelimbs was employed as measures of the intensity of the rigidity. In addition to the rigidity developed in forelimbs (tonic component), phasic tension (phasic component) was induced by mechanical stimulation given to the hindlimbs. On the basis of the effects on the tonic and phasic components of rigidity, the drugs examined were classified into three types of groups : (1) drugs which depressed both tonic and phasic components (mephenesin and tolperisone), (2) drugs which depressed tonic and augmented phasic components (orphenadrine and chlorpromazine), and (3) drugs which depressed phasic but had almost no effects on the tonic components (diazepam, nitrazepam and clonazepam).