[Animal models for junction-type nevi could be helpful for research into the histogenesis of nevus cell nevi and malignant melanoma.] Albino guinea pigs [70] were used in the experiment to investigate the influence of the carcinogen, 9,10-dimethyl-1,2-benzanthracene on amelanotic melanocytes. A field 5 cm2 was marked on the flank of each animal. Hairs from these fields were clipped twice a week and painted for 20 consecutive wk with 0.3 ml of 1% 9,10-dimethyl-1,2-benzanthracene in acetone. During the 1 yr observation period, pigmented spots appeared in 40 animals. Biopsies were taken under local anesthesia, and sections were prepared for light and EM observations. Melanin formation of unknown mechanism took place in the epidermal melanocytes and in the melanocytes from the outer root sheath. These melanocytes also formed junctional and compound nevi; serial sections revealed various stages of pigmented nevi development. Schwann cells did not participate in the formation of nevi. Evidence is presented that 9,10-dimethyl-1,2-benzanthracene can convert amelanotic melanocytes into melanin-producing cells in the albino guinea pig skin. In addition, this system produces an animal model for the development of junctional and compound nevi.