In an investigation into the transformation of mossy fiber input to Purkinje cell output in the cerebellar cortex, we have developed a network model including a sophisticated compartmental model of the Purkinje cell. Analysis and simulations demonstrated the need to include a large number of parallel fibers (244 000) to obtain realistic Purkinje cell firing patterns for random mossy fiber input. Using smaller numbers of parallel fiber inputs resulted in unrealistic Purkinje cell spiking characterized by spontaneous dendritic calcium spikes. Because the scale required is beyond workstation capabilites, we used a 128-processor Cray T3E running PGENESIS. This size network could produce realistic spiking patterns provided the stellate cell inhibition was tuned carefully.