Spontaneous basophilic hypertrophic foci of the parotid glands in rats and mice.

Basophilic hypertrophic foci of the parotid glands characterized by focal hypertrophy and hyperchromasia of acinar cells was seen in Sprague-Dawley rats and Swiss mice with overall incidences of 4.8% (102/2,138) and 0.6% (4/723), respectively. In rats, the incidence increased significantly with age, but the size and number of the lesions per rat did not. Based on the similar incidence in both control and treated animals, lesions were considered to be spontaneous. Their morphology and growth pattern suggested that they were neither degenerative, necrotic, hyperplastic/preneoplastic, nor neoplastic, but, that they are a distinct pathologic entity of a nature not yet determined.