The recovery cycle of amplitude and latency of the P100 of the chequerboard pattern reversal visual evoked response was studied in 16 normal subjects and 20 patients with multiple sclerosis. There was no clear-cut pattern of recovery with respect to amplitude but at interstimulus intervals of less than 40 msec the latency of the test response P100 tended to be significantly delayed, the magnitude of this delay being virtually constant at intervals of 20 msec or less. Only 8 of 40 eyes from the 20 multiple sclerosis patients showed a pattern of recovery of latency significantly different from the normal subjects. Possible mechanisms for these observations are discussed with particular reference to current concepts of the pathophysiology of multiple sclerosis.