The laterodorsal tegmental nucleus (LDT) is the largest aggregation in the brainstem of cholinergic neurons whose axons reach the thalamus as part of a diffuse projection to the forebrain. We measured the regional blood flow in the thalamus by means of laser Doppler flowmetry, and examined whether the blood flow was regulated by the ascending cholinergic nerve fibers originating in the LDT. Experiments were performed on urethane-anesthetized rats whose upper cervical spinal cord was transected to avoid response of systemic blood pressure following LDT stimulation. The ascending cholinergic nerve fibers were excited by electrical or chemical stimulation applied to the LDT. The regional thalamic blood flow increased in response to repetitive electrical stimulation and chemical stimulation with L-glutamate to the LDT. The response, starting several seconds after the onset of electrical stimulation and lasting as long as 1 min, was reduced by i.v. scopolamine, a cholinergic muscarinic receptor antagonist. The results indicate that regional blood flow in the thalamus is increased by excitation of the ascending cholinergic nerve fibers originating in the LDT mainly through the cholinergic muscarinic receptors.