In contrast to some previous reports suggesting a delay in synapse formation in vitro, we found that under ideal conditions, most hippocampal and hypothalamic rat neurons were synaptically coupled after 3 or 4 days in vitro. Synaptophysin immunocytochemistry revealed strongly stained presynaptic boutons by 3 days in vitro. Studies with time-lapse laser confocal imaging of FM1-43 revealed that axonal boutons were recycling their synaptic vesicles, an indication of synapse formation, as early as 3 days after plating. To test the hypothesis that neurite outgrowth was enhanced in high-density cultures, thereby increasing the probability of synapse formation, neurons were transfected with the jellyfish green fluorescent protein (GFP) gene. After 2 days in high-density cultures, green fluorescent neurites were about three times longer than in sister neurons plated in low-density cultures. Even in single dishes, GFP-transfected cells in contact with other neurons had neurites that were at least three times longer and grew faster than more isolated cells. Neurons grew longer neurites (+51%) when growing on surface membranes of heat-killed neurons than on polylysine, underlining the importance of plasma membrane contact. Calcium imaging with fura-2 and whole cell recording showed that both GABA and glutamate presynaptic release occurred after 3 or 4 days in vitro in high-density cultures but was absent in low-density cultures at this time. Together, these morphological, cytochemical, and physiological data suggest that the distance an axon must grow to find a postsynaptic partner plays a substantial role in the timing of synapse formation. Although other factors in vitro may also play a role, the distance to a postsynaptic target, which defines the interval during which an axon grows to its target, can probably account for much of the difference in timing of synapse formation previously reported in vitro. A short intercell distance may increase the concentration of limited amounts of trophic factors available to a nearby cell, and once contact is made, a neuronal membrane provides a superior substrate for neuritic elongation.