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Abstract. Chronic dermatosis developed in sparsely-haired, lightly pigmented, ventral body skin in 397 of 1 680 beagle dogs by the time the dogs were 5 ' 1 2 years old. Twenty-six of 55 other beagles had developed such lesions by 12 years of age. Squamous cell carcinomas developed in the sites of dermatosis in eight of the 397 younger and five of the 26 older beagles. The lesions resembled solar keratosis (solar dermatosis, actinic dermatosis or senile keratosis) in man. They developed under circumstances suggesting that solar radiation is involved in the pathogenesis, as it is in man.Chronic dermatosis and keratosis occur in human skin inadequately protected from prolonged exposure to sunlight. The changes progress to invasive carcinoma often enough to be considered precancerous [ 7 ] . Chronic inflammatory and proliferative lesions also occur in the skin of dogs, but with the exception of nasal solar dermatitis, neither location nor circumstances of development have implicated chronic solar radiation as a cause, and carcinoma is not a sequel [13]. This report deals with the development of inflammatory and proliferative lesions in nonpigmented and sparsely-haired abdominal, inguinal, preputial, and scrota1 skin of beagles. The circumstances suggest that solar radiation is involved in the pathologic processes. Further, invasive squamous cell carcinomas have developed frequently enough in the abdominal, inguinal, and preputial sites of chronic dermatosis to indicate that the preexisting changes are precancerous and that this change may be considered a canine counterpart of human actinic dermatosis and subsequent neoplasia. Materials and MethodsThe clinic and pathology records of 858 male and 882 female purebred beagles were reviewed for data o n inflammatory, proliferative, or neoplastic lesions in nonpigmented skin. The beagles were part of a large colony in a research laboratory in rural northern Colorado. Most of these dogs will live out their lives in the colony and at the time of the study were from 3 to 12 years old. One thousand three hundred and twenty designated "long-term" beagles were exposed to 20 or 100 R (51.6 x 10F or 258 x 10V4 Cikg) whole body, "'Co gamma radiation. Three hundred and sixty "long-term" control dogs were sham irradiated. Fifty-five nonirradiated beagles, the F, breeding group, were the oldest survivors of original breeding stock in the colony. Five other beagles, designated "short-term", were included because cutaneous squamous cell carcinomas developed (table I ) . All beagles were maintained in 218

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