Lambs of various ages (4-30 days) were monocularly deprived for periods ranging from 4 to 51 days after which they were prepared for conventional electrophysiological recording from the striate cortex. Sufficient units were sampled to obtain a representative ocular dominance (OD) histogram. A second sample of units was then obtained after the lambs had been given an i.v. dose of 4-aminopyridine (4-AP; 0.5-5.3 mg/kg) which increases synaptic transmission. For four out of five lambs in which the deprivation had been of 4-5 days duration there was a significant increase in responsiveness of the deprived eye to stimulation after the 4-AP had been given. By contrast, only one out of four lambs which had been deprived for 19-51 days showed a significant recovery after 4-AP treatment. The results suggest that during the initial stages of monocular deprivation the deprived eye remains connected to, but is less effective in driving, cortical cells. One explanation of the failure to reactivate the deprived eye after long periods of deprivation is that the deprived eye becomes anatomically disconnected from cortical cells.