The electrical stimulation of the caudate nucleus could induce a caudate spindle in the rat as in case of the cat and monkey. The spindle was enhanced by atropine (3-10 mg/kg, i.v.) or morphine (3-10 mg/kg, i.v.) and the effect of these drugs were completely abolished by eserine (0.3 mg/kg, i.v.), but not by methysergide (1 mg/kg, i.v.), PCPA (400 mg/kg, i.p. for 3 days) or reserpine (3 mg/kg, i.p. for 2 days). An intraventricular injection of atropine (5-10 microgram/rat) or morphine (10 microgram/rat) exerted a facilitatory action on the spindle, while an intrathalamic and intracaudate injection of atropine (5-50 microgram/rat) showed a suppressive action. The spindle was also markedly enhanced by the microinjection of atropine (15-30 microgram/rat) or morphine (30 microgram/rat) into the reticular formation, and these actions were completely antagonized by eserine (0.3 mg/kg i.v.), but only partially by methysergide (4 microgram/rat, intraventricular administration.) These results suggest that the cholinergic system may play an important role in the regulation of the caudate spindle.