It is known that neurons in the nucleus of Forel and the central tegmental fasciculus are active during shivering. In this study, bilateral lesions were placed in either of these 2 regions in 35 cats. Their ability to maintain normal rectal temperature in response to external cooling was tested at intervals up to 3 weeks or more. In 10 of these cats, a test was made of the ability to raise rectal temperature from 32° C to normal by shivering before and after the lesions. In six cats, quantitative measures of oxygen consumption rates were made during cold stress before and after the lesions. The results indicate that the lesions did not impair the shivering response, except in cats with postoperative respiratory infections or diminished food intake, and then only after the development of cachexia.