Primary visual cortex (V1) is the site at which right and left eye pathways are first integrated, leading to a representation of the visual world in depth. The ocular dominance (OD) of individual cortical neurons varies and may be changed by altering visual experience during the developmental critical period. Estimates of OD, commonly used to quantify the right and left eye synaptic inputs, have previously been based on spike rate. Membrane potential (V(m)), however, is more closely related to the synaptic inputs onto neurons and should therefore more closely reflect the degree of input from the two eyes. To determine the relationship between OD based on membrane potential and on spike rate, intracellular recordings were made from visual cortical neurons. OD based on spike rate was systematically more monocular than OD based on membrane potential. The discrepancy between membrane-potential OD and spike-rate OD may be accounted for by a simple model of V(m)-to-spike-rate transformation.