This study measures the effect of a vestibular stimulation program on the stereotypic rocking behavior of three severely mentally retarded persons within both experimental and natural settings. A multiple baseline design was used. Frequency and duration of stereotypic rocking behavior were measured by a partial-interval time sample. Results indicated a statistically significant reduction of both frequency and duration of rocking behavior directly after receiving vestibular stimulation and 1 hour after stimulation. The rocking behavior remained reduced after 6 days without the vestibular stimulation program. It was concluded that vestibular stimulation resulted in a reduction of the stereotypic rocking behavior of these subjects.