Peak angular saccadic velocities were measured during 30 degrees saccades in 18 patients with internuclear ophthalmoplegia who had full or nearly full adduction range, and 25 normal subjects. The following observations were made: 1. In the normal group adduction velocities were significantly faster than abduction velocities and centring movements were faster than decentring movements. 2. In patients with internuclear ophthalmoplegia adduction movements were significantly slower than in normal subjects and were significantly slower than abduction velocities. 3. Abduction velocities in patients with internuclear ophthalmoplegia were significantly slower than abduction velocities in normal subjects. It was concluded that measurements of peak angular velocities during saccades may be useful in detecting internuclear ophthalmoplegia or confirming the presence of suspected internuclear ophthalmoplegia. Slow abduction suggests that many patients with internuclear ophthalmoplegia (60 per cent in this series) have lesions affecting the supranuclear pathways subserving horizontal conjugate gaze.