Laminar field potential analysis of the cerebellocerebral and thalamocortical (T-C) responses was carried out in the cerebral cortex of squirrel monkeys (Saimiri sciureus) under pentobarbital anesthesia. Anatomical studies using horseradish peroxidase (HRP) were combined with electrophysiological studies. Stimulation of the fastigial nucleus induced bilaterally surface positive-depth negative potentials, i.e., deep T-C responses, in the medial part of the motor cortex and in the parietal cortex (mainly area 5) with a latency of 4 to 5 ms. Stimulation of the interpositus and dentate nucleus evoked contralaterally surface negative-depth positive potentials (superficial T-C responses) in the intermediate and lateral part of the motor cortex and in the premotor cortex (area 6) with a latency of 3 to 4 ms. Stimulation of the thalamic nuclei as well as the HRP study revealed that the medial part of V.o.p. (posterior basal part of VL) and Z.o (upper part of VL) and the intralaminar nucleus (CL) mediate the interpositus- and dentate-induced responses to the motor and premotor cortex, and that the ventrolateral part of V.o.p. and Z.im (upper part of VPLo) relays the fastigial-induced responses to the motor cortex.