1. With direct electrical excitation of the radial and sciatic nerves, and of the forefoot and hindfoot by means of superficial electrodes, the depth profile of field potentials in 59 marked tracks and the activity of 207 single Purkinje cells in the cerebellum of curarized grassfrogs were measured. 2. The depth profile of the polarity of field potentials following somatic excitation of mossy fibers (MF) and climbing fibers (CF) corresponded to that in the cat: CF field potentials were negative in the molecular layer and positive in the granular layer. MF excitation produced an N2 potential in the granular layer and an N3 potential in the molecular layer (Fig. 2). 3. A somatotopic distribution of the MF-induced responses following somatic stimulation could be determined neither for field potentials nor for recordings from single cells. 84% of the Purkinje cells responded with MF action potentials to stimulation of two or more nerves. CF-responses of single Purkinje cells and CF field potentials following nerve stimulation appeared predominantly ipsilaterally in the ventral half of the cerebellum; a few responses lay in a narrow contralateral zone (200–400 μm lateral to the midline). Following stimulation of the foot, no contralateral CF-responses were found. 64% of the Purkinje cells responded only to stimulation of one of the nerves (most often the ipsilateral radial nerve), while 36% responded to stimulation of two or more (primarily ipsilateral) nerves (Figs. 8,9). 4. For stimulus intensities up to twice threshold, the latency histogram of the ipsilateral MF-responses of Purkinje cells exhibited two maxima with mean values at 36 msec (s, ±10 msec) and 76 msec (s, ±11 msec). The latency histogram for contralateral MF-responses lacked the second maximum (Fig. 6). 5. Stimulation of the eye by light produced excitation of Purkinje cells by way of both mossy fibers and climbing fibers. In 12 Purkinje cells (6%), MF-responses were observed. Of these, five cells had an on-response (mean latency, 138 msec;s, ±17.4 msec) and seven cells had an off-response (mean latency, 89 msec;s, ±13.8 msec; Fig. 13). CF-responses of the “on” or the “off” type appeared in seven and four Purkinje cells, respectively. On-responses had extremely variable latencies, between 200 and 500 msec (mean range of variation for any individual cell: 70 msec), while off-responses had a mean latency of 87 msec with very little scatter (s, ±6.3 msec; Figs. 14, 15). 6. Interaction of CF-responses was studied in three different conditions: with paired visual and somatic stimuli, with alternate stimulation of different somatic nerves, and during spontaneous discharge. In only one case was the interval between two CF-induced discharges of a Purkinje cell smaller than 200–300 msec (Figs. 10,16). 7. A comparison of the anatomical and functional properties of the climbing-fiber systems of cat and frog suggests that the neuronal interconnections within the two systems are similar.