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The relationship between the size of parasympathetic neurones and the number of synaptic boutons terminating upon them has been studied in the cardiac ganglion of Xenopus laevis. Synaptic boutons were visualized by impregnation with zinc iodide and osmium (ZIO), which by electron microscopy was shown to stain heavily all synaptic boutons in six preparations. Light microscopic examination of the unipolar ganglion cells in intact tissue reveals that larger neurones have more synaptic boutons. The number of boutons terminating on the cell body is significantly correlated with its surface area. By statistical means it was possible to demonstrate that the relation between bouton number and surface area is linear and that the regression line has a y-intercept not significantly different from zero. Therefore the density of synaptic boutons, one per 127 micron2 of cell body surface, is independent of cell size. The size of synaptic boutons, measured as the area of apposition between bouton and cell body, is similar for small and for large ganglion cells; thus a constant fraction (2%) of the cell body on average is covered by synaptic boutons, regardless of cell size. The correlation between bouton number and cell body surface area is not the result of interaction between boutons. The frequency distribution of boutons per normalized cell was found to be similar to that expected from the Poisson distribution. Thus the probability that a bouton will be 'assigned' to a particular cell is independent of how many other boutons are present. The only factor that appears to influence the number of boutons on the cell body is its size.