As a result of two operations which were performed on a rhesus monkey with the object of working out an approach for the destruction of the anterior nuclei of the thalamus, not only were the latter involved, but lesions were also produced in the frontal lobe. The lesions were largely caused by the removal of firm adhesions which had developed after the first operation. The motor disturbances and the mental changes which appeared after the second operation revealed some features which had much in common with a clinical syndrome previously observed in a patient who had died from a frontal lobe tumour in the same region. For this reason it seems worth while to record and to discuss these two cases. The experimental operations may be described in some detail in order to explain the extent of the lesions produced.