Spike potentials of single cerebellar neurons superimposed upon a baseline of typical cerebellar waves have been recorded from the cerebellar cortex in unanesthetized decerebrate cats using fine, penetrating wire electrodes and conventional oscilloscopic recording equipment. Localization of the tips of electrode wires left in situ was accomplished by microscopic examination of thick, fixed, stained and cleared blocks of cerebellar tissue. Low voltage wave activity and freedom from spike potentials characterized recordings from the molecular layer and white matter. Higher voltage waves, occasional spike potentials, and massive injury discharges upon movement of the electrode characterized recordings from the Purkinje cell and granule cell layers. These findings indicate the great localizing ability of the wire electrode and the origin of the cerebellar waves from the Purkinje and/or granule cell layers. Temporary cerebellar ischemia or administration of ether produced abolition of spike potentials before wave activity was affected. This change was reversible, wave activity recovering before spike potentials. Summation of spikes of the type recorded from these electrodes cannot therefore be considered as a source of cerebellar waves. Arguments are presented in favor of the hypothesis that cerebellar waves may be due to fluctuations of membrane potential not directly associated with conducted nerve impulses.