Abstract
Brown fat tumours, 'hibernomas', are described in 2 Wistar and 1 CD rats; 2 were intrathoracic and 1 was interscapular. The incidence of hibernoma in rats undergoing carcinogenic bioassay at this laboratory over the 4 years between 1977-1981 is 3 in 3200. Clinical and autopsy findings and histological and ultrastructural observations are reported, and a review of the literature is presented.