AbstractMorphological and electrophysiological studies of the agranular cerebellar cortex of ferret after infection with panleukopenia virus have led to the following conclusions:A.At light microscopic level this agranular cerebellar cortex is shown to be totally disorganized when compared to its normal lamination.B.From an ultrastructural point of view, Purkinje cells are present throughout the thickness of the cerebellar cortex, demonstrating dendritic branches studded with unwed spines.C.Purkinje cells receive four different types of inputs‐climbing fibers, mossy fibers, stellate cell terminals, and Purkinje cell axon collateral terminals. The mossy fiber contacts which are generally not present in mammals are made in some cases to elongated, “dolichoderic” spines.D.Stellate, basket, and Golgi cell interneurons are present, and receive inputs from mossy and climbing fiber terminals, and probably from axon collaterals of Purkinje cells. The mossy fiber input to these neurons represents a true reorganization of the cerebellar circuit, since it has never been observed in any other vertebrate.E.Stellate and basket cells terminate in contact with dendrites and somata from Purkinje cells, while the Golgi cell terminals seem to be restricted to somata and dendrites of other Golgi cells.F.Electrophysiologically, Purkinje cells can show normal excitability following antidromic invasion, and may be activated by the mossy and climbing fiber afferent systems. Intra‐and extracellular recording from Purkinje cells demonstrates that the mossy fiber afferent system activates Purkinje cells with a latency of approximately 1 msec and its excitatory action is graded with relation to the amplitude of the white matter stimulation. Following an initial excitation, this input generates a longlasting inhibition (50 msec), which is attributed to mossy and climbing fiber activation of inhibitory interneurons.G.Climbing fiber activation generates the typical all‐or‐none burst response in Purkinje cells extracellularly. Intracellularly, large unitary EPSPs characteristic of this form of activation may be recorded with a latency of 1.5 to 3 msec. This large unitary EPSP seems to behave in the usual one‐to‐one relation (one climbing fiber to one Purkinje cell).H.Double climbing fiber activation reveals that a conditioning white matter stimulation produces a total inhibition of a climbing fiber burst at 15 msec interval, which indicates a rather strong inhibitory action on Purkinje cells following this form of activation.I.Climbing fiber activation is followed in most cases by a so‐called climbing fiber reflex, suggesting that the inferior olive in this agranular condition is functioning in a normal manner.J.Finally, some of the neurobiological implications of these findings are considered