1. Skin cancer in rats (and in mice) can be produced by exposing them daily for definite periods to ultraviolet light.
2. Roffo produced identical neoplasms in rats by exposing them daily for 5 hours to sunlight. This was confirmed in the experiments at Onderstepoort, where rats were daily exposed for 4.5 hours to sunlight. In one instance a new growth developed in a rat exposed daily for only 2.5 hours.
3. So far no spontaneous skin cancer have been observed in the rats at Onderstepoort when kept under sheltered conditions but in one rat in these experiments, a fibroblastic sarcoma was found in the liver, lung, and omentum.
4. The skin cancers occurred on the hairless parts of the skin, and in the majority of rats new growths appeared in more than one place on the body. The ears were most frequently affected.
5. There was no definite occurrence of metastasis into other organs.
6. The earliest appearance of a new growth, to which attention is drawn, was ±10 months after exposure of the rats to sunlight.
7. In all the new growths a microscopical diagnosis of an epidermoid carcinoma was made. The statement by Roffo that some of these neoplasms were of the nature of sarcoma could not be confirmed at Onderstepoort.
8. The earlier lesions revealed the usual characteristics of an acanthoma associated in some cases with hyperkeratosis, while in the large actively growing neoplasms there was much evidence of anaplasia.