Synapse formation was followed in dispersed cell cultures of rat cerebellums stained either by osmium-uranyl-lead (Os-UL) or by ethanolic phosphotungstic acid (E-PTA). The numerical densities of synapses stained by Os-UL were always significantly higher than those stained by E-PTA (from 3 to 35 days in vitro). This difference suggests that some portion of the populations of both immature and mature synapses was not stained by E-PTA. The width of the synaptic cleft (28.4nm) in synapses stained by E-PTA was only 9nm more than that of the cleft in synapses stained by Os-UL (19.4nm), suggesting that some portion of one or both of the synaptic membranes is stained by E-PTA. Analysis of data from 7 other staining procedures utilizing both ethanolic and aqueous solutions demonstrated that the differences in cleft width described above appear to be due to the various affinities of the stains for different portions of synaptic membranes, and do not represent differences due to shrinkage artifact. In examining the parameters of synaptic structure during development of the cultures, a statistically significant increase in both the height and width of the presynaptic dense projections was found. Changes in synaptic morphology during synaptogenesis in this culture system were similar to those reported for the cerebellar cortex in vivo.