The ultrastructural appearances of preneoplastic liver nodules induced in male Wistar rats through sequential treatment with diethylnitrosamine (DENA) and 2-acetylaminofluorene (2-AAF) followed by partial hepatectomy were investigated. Although separately non-specific, the lesions reported represented altogether a fairly characteristic pattern consistent with previous observations in tumors and in preneoplastic nodules induced by long-term treatment with single carcinogens. Such data support the view that sequential treatment followed by partial hepatectomy is a reliable method to study early phenotypic changes in hepatocarcinogenesis. Such treatment is also advisable because it requires only a short period of latency in comparison to other in vivo screening tests of potential carcinogens.