Significant changes in aldehyde dehydrogenase (ALDH) activity occur during chemically induced rat hepatocarcinogenesis. We have developed a procedure for the histochemical localization of hepatic ALDH which has proven extremely useful as an additional probe for studying changes in this enzyme during hepatocarcinogenesis. Frozen sections of fresh tissue were stained for ALDH using either propionaldehyde-NAD to detect normal liver ALDH or benzaldehyde-NADP to detect tumor ALDH. Histochemically, normal liver ALDH activity is strongly centrilobular with only slight periportal activity and produces a characteristic staining pattern. During hepatocarcinogenesis, ALDH staining patterns in grossly normal liver range from normal-appearing to patterns of distinct, intense focal hepatocyte staining with propionaldehyde-NAD and/or benzaldehyde-NADP. ALDH-positive foci are found both in normal regions of tumor-bearing livers and prior to the appearance of gross neoplasms. Neoplastic nodules and carcinomas possess a wide variety of ALDH staining patterns between and within lesions. Neoplasms with elevated ALDH activity with propionaldehyde-NAD and/or benzaldehyde-NADP, as well as with no detectable ALDH, have been observed. Changes in ALDH can be identified histochemically at a time in hepatocarcinogenesis when other analytical methods cannot detect significant changes. Moreover, considerable heterogeneity in expression of tumor ALDH is demonstrable by histochemistry.