Monkeys wore either base-out prisms to promote changes in tonic vergence or periscopic spectacles to promote changes in the coupling between accommodation and vergence. Eye movements were recorded using the magnetic search coil technique and the monkeys were rewarded for accurate fixation. Two normal monkeys and two monkeys which had previously received lesions of the flocculus and ventral paraflocculus were studied. After 30 min of prism viewing the two normal monkeys had elevated phoria--increased by approximately 50% of the prism stimulus. The two lesioned monkeys also had phoria increases of a similar size after prism viewing. The effect of duration of prism viewing on the magnitude and time course of phoria elevation was studied in one normal monkey. The initial magnitude of phoria elevation and the time constant of relaxation of phoria both increased as the duration of prism exposure varied from 5 s to 30 min. Initial phoria increased approximately in proportion to the logarithm of duration of prism exposure whereas the time constant of phoria relaxation increased linearly with duration. In the same normal monkey it was shown that increasing vergence by means of accommodative-vergence did not induce phoria changes. The effect of periscopic spectacle viewing was studied in all four monkeys. After 30 min of periscopic spectacle viewing all four monkeys had a higher AC/A ratio. The magnitude of the changes was (paradoxically) greater in the two lesioned monkeys than in the one normal monkey studied fully.(ABSTRACT TRUNCATED AT 250 WORDS)