The effects of transdermally administered scopolamine (TTS-scopolamine) (release rate 5 micrograms/h, one and two patches) and dimenhydrinate (100 mg) on caloric, angular acceleration induced and optokinetic nystagmus were examined in 16 volunteers in a randomized double-blind study. All drugs induced a statistically significant decrease in maximum velocity of caloric nystagmus, as compared with placebo. In the rotatory test, two TTS-scopolamine and dimenhydrinate reduced the vestibular gain significantly. No changes were observed in time constant. In the optokinetic test, all drugs tended to reduce the responses, but a statistically significant reduction was found only after two TTS-scopolamine. The results indicate that the drugs effective against motion sickness reduce the nystagmic response, which at least partly explains the mode of action of the drugs. The target organ of the drugs is presumably the vestibular nucleus, where vestibular and visual impulses are integrated to ensure optimal gain for vestibular orientation reflexes.