Sequential stimulation of the granular layer of the cerebellar cortex in vitro using 11 linearly aligned stimulating electrodes leads to massive population activity in the parallel fiber system and to spike activity in Purkinje cells (Heck, D., Neurosci. Lett., 157 (1993) 95-98; Heck, D., Naturwissenschaften, 82 (1995) 201-2030). The induced parallel fiber activity, however, might have been a result of direct stimulation of parallel fibers themselves and not of stimulation of granular cells or their ascending axons. We report here that using sequential 'moving' stimuli and varying the distance covered by the 'movement', parallel fiber population spike amplitude increases with distance and saturates for distances longer than 1.0 mm. This effect cannot be explained if parallel fibers are directly stimulated, but requires stimulation of the granular cells or their ascending axons. We conclude that the population spike activity and Purkinje cell responses induced by sequential stimulation of the granular layer of the cerebellar cortex slices in this and earlier experiments consists of orthodromic parallel fiber spikes.