Perrotatory neuronal response of tonic vestibular type I and type II neurons in the medial vestibular nucleus in the cat was stereotaxically recorded before and after the administration of dimenhydrinate (Dramamine) or diazepam (Valium). Response frequencies were calculated with an on-line instrument computer and changes in the frequencies were evaluated by statistical methods. Dimenhydrinate showed no significant difference in the perrotatory response between preinjection and 30 minutes postinjection of each of three dosages (2 mg, 8 mg, and 20 mg/kg). Diazepam at level of 0.4 mg/kg produced significant depression of the perrotatory response of the medial vestibular nucleus.