Neuronal coupling by gap junctions is common during early development of the brain. Coupling is thought to create functional cell assemblies which may be involved in the functional specification of brain areas and the formation of synaptic circuits. In the present study we used slices from the visual cortex of postnatal ferrets to investigate the temporal relationship of gap junction coupling and formation of functional synapses. Individual neurons were filled with the gap-junction-permeable dye biotin ethylenediamine while spontaneous synaptic currents were recorded using whole-cell patch clamp recording techniques. We found that dye coupling increased during the first 2 postnatal weeks resulting at a peak around P14, after which coupling steadily decreased until adult levels were reached in animals older than P30. Spontaneous synaptic activity increased 30-fold between birth and maturity (from 10.8 +/- 2.4 to 318 +/- 54 events/min). The sharpest rise in synaptic activity, an over 5-fold increase, occurred between P15 and P19, shortly after the invasion of thalamocortical fibers.