In an attempt to study the neural mechanism of the development of the caudate spindle, effects of dopamine agonist or antagonists and baclofen on the spindle were studied in gallamine-immobilized rats. The spindle was significantly enhanced by chlorpromazine, haloperidol and pimozide, and these effects of drugs were completely antagonized by apomorphine. A marked prolongation of the duration of the spindle was observed by the treatment with baclofen, and this side effect was antagonized by apomorphine but not by methysergide, eserine or picrotoxin. These results suggest that the dopaminergic system may play an important role in the development of the caudate spindle in the rat, i.e. as a negative modulatory system in the manifestation of the caudate spindle.