Organotypic cerebellar cultures derived from newborn mice were exposed to cytosine arabinoside for the first five days in vitro to destroy granule cells and functionally compromise glia. Such granuloprival cultures undergo a circuit reorganization featured by Purkinje cells sprouting recurrent axon collaterals that hyperinnervate other Purkinje cells. Intracellular recordings were used to compare the electrophysiological properties of Purkinje cells in granuloprival cultures to those of Purkinje cells in standard cultures. Purkinje cells in granuloprival cultures have similar membrane potentials to those of Purkinje cells in standard cultures, but have a lower input resistance. A reduced input resistance could affect the effectiveness of inhibitory synaptic input. Intracellular recordings from Purkinje cells of standard cerebellar cultures between 13 and 21 days in vitro exhibit spike activity consisting of a mixture of complex and simple spikes. The complex spikes contain a fast rising action potential followed by a depolarizing potential on which a plateau and several spike-like components are superimposed. This type of activity has been observed in mature Purkinje cells in vivo and in vitro. By contrast, at resting membrane potential Purkinje cells in granuloprival cultures have simple spike activity reminiscent of the type of activity seen in immature Purkinje cells, while at hyperpolarized potentials they generate complex spikes. These observations indicate differences in the expression of intrinsic electrophysiological properties underlying complex spike generation between Purkinje cells of organotypic and granuloprival cerebellar cultures. Our results illustrate the considerable plasticity of Purkinje cells in the presence of altered neuronal circuitry. In the absence of normal excitatory input, their spontaneous activity is regulated by intrinsic membrane properties.