A liposome model of exocytosis has been used to examine the nanometer environment of the space between the electrode and the membrane. First, this has been used to test models of coulometric efficiency for different size vesicles with 5- and 33-microm electrodes. The resulting model has a best fit that suggests that the liquid space is approximately 300 nm across the gap. Given this dimension, the volume of the electrode-membrane space is not large enough to accommodate the volume of larger vesicles in cells such as the mast cells of the beige mouse. Second, the model suggests that flow of solution from the exocytosis event is more important than diffusion. Flow from the finite vesicle volume past the electrode leads to less charge passed. Third, and finally, this system can be used to model transport in the synapse and so it is possible to examine the idea that transmitter flows in addition to diffusing from the synapse. This model should be useful in understanding and quantifying the escape of transmitter from synapses in vivo.