Experiments were performed in decerebrate cats to find out whether the noradrenergic afferent system to the cerebellar cortex modified the gain of the vestibulospinal (VS) reflexes and, if so, whether this effect was mediated through beta-adrenoceptors. Unilateral injection into the vermal cortex of the culmen of the beta-adrenergic agonist isoproterenol (0.25-0.50 microliter at 8-16 micrograms/microliters of saline) increased the gain of the ipsilateral and to a lesser extent also of the contralateral triceps brachii to roll tilt of the animal at 0.15 Hz, +/- 10 degrees. The opposite result was obtained after local injection of an equal dose of the beta-adrenergic antagonist, propranolol. Only slight changes in the phase angle of the responses were obtained. The effective sites were located within the parasagittal zone B which upon stimulation inhibited the activity of the ipsilateral triceps brachii. Since most of the Purkinje (P)-cells of this corticocerebellar area, which project to the lateral vestibular nucleus, fire out-of-phase with respect to the related VS neurons, we postulate that the beta-adrenergic system enhances the amplitude of the P-cell responses to labyrinth stimulation, thus increasing the gain of the VS reflex.