1. The ophthalmic branch of the trigeminal nerve (V1), which carries extraocular proprioceptive afferents, was sectioned unilaterally or bilaterally in kittens and adult cats. Depth perception was measured behaviorally in these sectioned cats, as well as in control cats. 2. For kittens that underwent unilateral V1 sections at 6-11 wk of age, postsurgical values of binocular depth perception--measured 1.5-3 mo later--were 2-3 times worse than in normal control animals. Cats that underwent unilateral V1 sections as adults, however, showed no postsurgical deficits in binocular depth perception. 3. For kittens that underwent bilateral V1 sections at 6.5-7.5 wk of age, similar longterm impairments were found in binocular depth perception. No impairment was found in two kittens bilaterally sectioned at 11.5 wk of age. A cat that underwent bilateral sections as an adult also showed no binocular depth perception deficits. 4. Although these behavioral effects were observed only when unilateral and bilateral V1 sections were performed up to a certain age in development, they differed in two ways. 1) Imbalance of extraocular proprioceptive inflow produced by unilateral section had a deleterious effect at an age when the final adult level had been reached. At that stage, complete suppression of inflow produced by the bilateral section failed to impair the final level of binocular performance. 2) Short-term effects observed during the week following the section appeared in bilaterally operated animals as a transient freezing of the presurgical binocular performance whatever the age of the section during the sensitive period. In contrast, short-term effects produced by unilateral section were found to be age dependent: a progressive slowing down in the normal rate of improvement of binocular thresholds was observed following a section performed at 5 wk of age; an arrest in development was found when surgery was done at 6-7 wk of age. A significant impairment appeared within 2 days when the section was performed at 11 wk of age. 5. In all experimental kittens, monocular depth perception thresholds were unaffected or impaired only to a minor extent (less than 15% change) following the unilateral or bilateral section. In unilaterally operated kittens, there were no consistent differences associated with the side of the section. 6. A sham-operated kitten, in which the V1 was visualized but not cut, showed no impairments in binocular or monocular depth perception.(ABSTRACT TRUNCATED AT 400 WORDS)