Activation of vagal afferents reflexly inhibited locomotion induced by stimulation of the mesencephalic locomotor region in decerebrate cats. However, this reflex has not been tested in intact mammals. Therefore, the purpose of this study was to test the hypothesis that vagal afferent stimulation would inhibit somatomotor activity in the intact conscious rat. Six Sprague-Dawley rats were chronically instrumented with carotid arterial and femoral venous catheters and electromyogram (EMG) electrodes inserted into the biceps femoris muscle. Cardiac autonomic efferent blockade [atropine methyl bromide (14 mg.kg-1, i.v.) and metoprolol (14 mg.kg-1, i.v.)] and alpha-adrenergic receptor blockade [phenoxybenzamine (5 mg.kg-1, i.v.)] was achieved to prevent bradycardia and hypotension. Vagal afferents were stimulated (phenyl-biguanide 2.5 and 5.0 micrograms.kg-1 i.v.) during steady state exercise (9.0 m.min-1, 10% grade). Phenyl-biguanide decreased exercise EMG activity 30 +/- 6% and 54 +/- 10% in a dose dependent manner without significantly altering mean arterial pressure or heart rate. We speculate that this reflex may serve as a negative feedback mechanism to indirectly reduce myocardial oxygen demands during exercise.