A method of producing decerebrate rigidity in cats by means of a radio frequency current was developed. Adequate control of the extent an shape of the lesions necessitated a preliminary investigation of the relationship between the intensity and duration of current, the size of the lesion, and the tissue impedance before and after destruction; this was performed on guinea pigs. The mesencephalic reticulum in cats was identified by depth microelectrode recording and electrical stimulation. Unilateral, midline, and combined stereotactic radiofrequency lesions were made. The anatomical location, the pathological characteristics, and the physiological significance of the lesions with regard to the production of decerebrate rigidity and of the other clinical reactions of the animals are discussed.