The carcinogenic effect of 2,2'-dihydroxy-di-n-propylnitrosamine (DHPN) was examined in the primitive primate, tree shrew (Tupaia glis). DHPN was administered at a dose of 250 mg/kg body weight sc once a week for 80 weeks. Between 65 and 102 weeks, 8 of 9 males given DHPN (89%) and 11 of 14 females given DHPN (78%) developed pulmonary adenomas. In 2 DHPN-treated males, in addition to adenomas, bronchioalveolar carcinomas were observed. Transmission electron microscopic examination of pulmonary adenomas from 4 DHPN-treated animals showed that Clara cells were the main components of these tumors. In addition to pulmonary tumors, 9% of the DHPN-treated animals developed squamous cell carcinomas of the skin and hepatocellular carcinomas. None of the 6 controls, which received olive oil alone, developed any tumors.