1,2-diacylglycerol (1,2-DAG) is involved in cell proliferation as an activator of protein kinase C (PKC) and has been shown to stimulate growth of cancer cells, raising the possibility of a role in tumor promotion. Ingested DAG oil, containing 70% 1,3-DAG and 30% 1,2-DAG, is digested and considered to be safe as edible oil. However, DAG may directly contact with oral cavity mucosa in undigested form. The present study was conducted to examine the effects of DAG oil on carcinogenesis in c-Ha-ras proto-oncogene transgenic (Tg) rats administered 4-nitroquinoline 1-oxide (4NQO, 10 ppm) in their drinking water for 10 weeks for initiation of mainly upper digestive organs. DAG oil added in basal diet at 5.5%, 2.75%, 1.38% and 0% with total fat made up to 5.5% with triacylglycerol (TAG) was administered during the initiation and post-initiation period. The study was terminated at week 12 (Tg females) and 20 (Tg males, wild females and males). The fatty acid composition of DAG oil was similar to TAG (linoleic acid 46.6% and oleic acid 38.9%). In Tg male rats, DAG oil administration was associated with significant increase (P<0.05) in the incidence of squamous cell carcinomas (SCC) of the tongue (5.5% DAG, 43.8%; 2.75% DAG, 20%; 1.38% DAG, 14.3%; 0%, 12.3%) with the Cochran-Armitage trend test and also number of tumors in coefficients for linear contrast trend tests. Tongue SCC induction of wild males and all females was not significant. The present results suggest that DAG oil may have enhancing and/or promotion potential for tongue carcinogenesis in male Tg featuring elevated ras expression.