The heterogeneous nature of vesicles has been studied by evaluating exocytotic events released from the cell body of a dopamine-containing neuron of Planorbis corneus. Vesicular exocytosis has been elicited by stimulation with in situ application of elevated potassium and has been monitored electrochemically with a carbon fiber microelectrode placed on the cell body. These electrodes allow individual release events to be monitored and quantitated to reveal vesicular dopamine content. Statistical analysis of individual release events demonstrates that two classes of vesicles with specific bimodal distributions in dopamine content and vesicle size are observed after cell stimulation. The effect of a psychostimulant on individual vesicular dopamine level has been studied by treating the cells with D-amphetamine. After a 20-min application of 10 microM amphetamine, changes in both vesicle content and size distributions are obtained with an overall decrease in vesicular dopamine level of 40%. The altered distribution show trimodal shapes indicating a third class of vesicles is created by amphetamine treatment. Our data appears to indicate that multiple classes of vesicles are released from the cell body of the dopamine-containing neuron and vesicular dopamine level can be manipulated by the application of the lipophilic weak base amphetamine.