Dark exposure has been reported to enhance plasticity in the adult visual cortex and to promote recovery from deprivation amblyopia both in rats [1] and in cats [2]. The physiological substrate of this recovery is as yet unknown. We employed optical imaging of intrinsic signals to monitor shifts in ocular dominance (OD) in the primary visual cortex (V1) of mice aged between postnatal day (P) 60 and P90. C57BL/6J mice were normally reared on a 12-h light/12-h dark cycle. Monocular deprivation (MD) by eyelid suture was carried out under isoflurane anaesthesia (2% in O2, 0.5 L/min). A control group of 7 non-deprived mice was imaged under general anaesthesia (1-2% of isoflurane in O2, 0.5 L/min, supplemented with 25 μg i.m. chlorprothixene as described previously to measure OD [3]. ECG and body temperature were monitored continuously. Animals were stimulated with horizontal white bars on a black background presented in the binocular visual field, drifting periodically across the screen at a rate of 0.125 Hz, with the stimulated eye determined by computer-controlled eye shutters. The magnitude of contra- and ipsilateral eye responses (C, I) was determined for each pixel within the V1 binocular zone by fast Fourier transformation, and an OD index (ODI) was calculated as (C – I)/(C + I). Compared with the control group, the ODI in 6 mice after 7 days of MD was shifted from 0.18±0.02 (mean ± SEM) to 0.04±0.01 (p < 0.001, t-test). We then re-opened the deprived eye under isoflurane anaesthesia and investigated the time course of recovery from MD. In a group of normally reared mice, 5 out of 7 animals (70%) recovered normal ODI values within 72 h (for all 7 mice, ODI=0.15±0.04, p=0.016). In contrast, 72 h of dark exposure immediately after the end of MD did not result in OD recovery; instead a further small OD shift towards the non-deprived eye was observed (ODI=0.08±0.01, n=7). However, when dark-exposed mice were returned to a normally lit environment, recovery of OD was accelerated. After only 36 h animals showed partial recovery of OD (ODI = 0.09±0.01, n=9, p=0.016), more prominent in the anterior segment of the V1 binocular zone; recovery was complete within 72 h (7 mice; ODI=0.19±0.01). In all groups, both the MD-induced OD shift and subsequent return to baseline were mediated by up- and down-regulation of ipsilateral (non-deprived) eye responses, with little change in deprived eye responses. These data suggest that brief periods of dark exposure, at least in mice, promote recovery from MD on the basis of the existing adult plasticity mechanisms.