The usefulness of rotatory testing (impulsive and sinusoidal) as an indicator of impaired horizontal semicircular canal function was evaluated in 63 patients with unilateral and bilateral decreased caloric responses. The rotatory stimuli were precisely controlled over a large magnitude range and EOG recorded nystagmus responses were quantified using digital analysis techniques. Rotatory testing was consistently abnormal in patients with complete unilateral caloric paralyses but was normal in over one-half of the patients with significant but less than complete unilateral caloric paralyses. The difference in maximum slow component velocity (SVMX) of induced nystagmus after the largest rotatory stimuli was the best indicator of unilateral impaired function. The patients with bilateral decreased caloric responses demonstrated three categories of rotatory response: 1. normal at all magnitudes of stimulation, 2. decreased but present after large magnitude stimuli, and 3. absent responses. It is concluded that although rotatory testing cannot replace caloric testing it can provide useful clinical information particularly in patients suspected of having bilateral vestibular disease.