Purkinje cells in zones a and b of the vermis and zone c in the intermediate cortex of the anterior lobe were intracellularly injected with horseradish peroxidase and the distribution patterns of the varicosities present on their axonal collaterals were quantified and compared at the light microscopic level. The data derived from this study reveal that each individual axonal arbor had a unique pattern of distribution. However, certain principles of distribution could be applied to the collaterals regardless of the cortical location of the parent cell, including the following: 1) the total number of varicosities derived from the axonal collaterals of individual Purkinje cells is relatively uniform; 2) the collateral plexi have a greater extent in the sagittal plane as compared to the transverse plane; 3) the majority of varicosities remain within 200-300 microns of the parent cell in both the sagittal and transverse planes; 4) there is a heterogenous distribution of varicosities within the area traversed by the axonal branches; and 5) the majority of varicosities are located within the Purkinje cell layer. Although there were similarities in the pattern of distribution for collaterals, there were also variations which distinguished the plexi in the three zones. The collaterals of zone a cells tended to be the most confined in both the sagittal and transverse planes. In contrast, several cells in zone b and c had branches that extended for relatively long distances in the sagittal plane. In zone b the collaterals have an asymmetric distribution around the cell of origin in the transverse plane. In zones a and c there is a tendency for a more symmetric pattern of distribution in this plane.(ABSTRACT TRUNCATED AT 250 WORDS)