The rules by which visual experience influences neuronal responses and structure in the developing brain are not well understood. To elucidate the relationship between rapid functional changes and dendritic spine remodeling in vivo, we carried out chronic imaging experiments that tracked visual responses and dendritic spines in the ferret visual cortex following brief periods of monocular deprivation. Functional changes, which were largely driven by loss of deprived eye responses, were tightly regulated with structural changes at the level of dendritic spines, and occurred very rapidly (on a timescale of hours). The magnitude of functional changes was correlated with the magnitude of structural changes across the cortex, and both these features reversed when the deprived eye was reopened. A global rule governed how the responses to the two eyes or changes in spines were altered by monocular deprivation: the changes occurred irrespective of regional ocular dominance preference and were independently mediated by each eye, and the loss or gain of responses/spines occurred as a constant proportion of predeprivation drive by the deprived or nondeprived eye, respectively. cortical circuits | ocular dominance plasticity | optical imaging | two-photon imaging E xperience-dependent plasticity in the primary visual cortex (i.e., V1) is a proving ground for understanding how the quality and quantity of input activity influences cortical synapses and circuits. In particular, blocking or reducing activity in one eye during a critical period of development leads to a physiological loss of responses to stimulation of the deprived eye and an increase of responses to the nondeprived eye (1, 2). The dynamics of ocular dominance (OD) plasticity are key to understanding its mechanisms. Initial estimates (2) suggested that functional changes in OD preference could occur with 3 to 6 d of monocular deprivation (MD). Subsequent experiments demonstrated that a saturating functional shift of OD preference required several days of MD at the peak of the critical period (3, 4). However, a significant reduction in deprived eye responses can be recorded even after a few hours of deprivation (5). Recent experiments have shown that the reduction of responses from the deprived eye occurs rapidly, followed by a slower increase of drive from the nondeprived eye (6, 7). Recovery of deprived eye responses after eye reopening occurs robustly (8, 9), although its time course has not been well studied.The structural basis of OD plasticity has also been studied at different timescales, but the mechanisms for rapid loss and recovery of responses remain unresolved. Several weeks of MD in cats leads to a shrinkage of deprived eye thalamocortical arbors and an expansion of nondeprived eye arbors (10, 11). However, 2 d of MD has no effect on the relative numerical synaptic density or synaptic vesicle protein density in deprived and nondeprived afferents (12). In contrast, 7 d of MD causes a decrease in the total length of deprived-eye geniculocortical arbors...