Vesicular catecholamine release has been measured amperometrically from undifferentiated rat PC12 cells using carbon fiber microelectrodes. During superfusion with high K+ saline, vesicular release was detected from similar to 50% of 200 cells investigated. On repeated stimulation the releasable pool of vesicles is rapidly depleted, while vesicle contents remains constant. Vesicular catecholamine release is not restored within 1 h after depletion of the releasable pool. Al-though the distribution of the cube root of vesicle contents of many cells is apparently Gaussian, maximum likelihood analysis of single cell data demonstrates double Gaussian distributions with median vesicle contents of 141 and 293 zeptomole. It is concluded that the releasable pool of vesicles in PC12 cells is heterogeneous. In the presence of L-DOPA mean vesicle contents increases, but cessation of release cannot be prevented, indicating that the number of releasable vesicles in PC12 cells is limited by a slow rate of vesicle cycling.