The understanding of the information processing performed by complex neuronal networks in the central nervous system will require techniques permitting the simultaneous monitoring of the electrical activity of neuronal ensembles. Voltage sensitive dyes offer the potential for non-invasive optical monitoring of the activity in large populations of neurons. In this report we describe the use of voltage sensitive dyes and image processing techniques to monitor in vivo the activation of parallel fibers and associated neuronal events produced by stimulation of the cerebellar cortex in the rat. Despite the temporal limitations of video processing a relatively brief set of neuronal events was successfully imaged. Using this methodology we demonstrate that the detected fluorescent light changes were highly correlated with the evoked extracellular field potentials. Graded surface stimulation produced graded spatial patterns consistent with known parallel fiber anatomy and physiology. The optical signals were dependent on the presence of the voltage sensitive dyes and were abolished by topical application of a local anesthetic agent. In essence, activation of a parallel fiber beam and associated activity were imaged at relatively high resolution.