Buffalo/Mna rats (Mna rats) with spontaneous thymoma show the motor dysfunction represented by an exacerbated fatigability of hind-limb muscles. In this study fast (extensor digitorum longus, EDL) and slow (soleus) muscles of these rats were examined physiologically and pharmacologically. ACI strain served as a reference. The mechanical fatigability of the EDL muscle during repetitive nerve and direct stimulation in Mna rats was greater than that in ACI rats, but electromyographical fatigabilities of both strains were quite similar. Smaller amplitude of evoked electromyograms, twitch tension (t.t.) and muscle weight (m.w.) and longer twitch relaxing time were observed in the EDL muscles of Mna rats as compared with ACI rats. On the other hand, the soleus muscles of rats of both strains showed similar t.t., and neither mechanical nor electromyographical fatigability. However, smaller t.t./m.w., greater m.w. and lower sensitivity to d-tubocurarine chloride were observed in the soleus muscles of Mna rats as compared with ACI rats. The motor dysfunction seems to result mainly from the muscle weakness and the myogenic fatigability in the fast muscle. The possibilities of some change at the site of neuro-muscular junction and a latent abnormality in the soleus muscle are also suggested.