Pancreases of Long-Evans rats were treated in vitro and in vivo with 3-methylcholanthrene and examined by histofluorescence techniques. For in vitro studies, tissue sections were dipped in 3-methylcholanthrene and incubated in vitro. These sections revealed that acinar and ductal epithelium had equal fluorescence which suggested equal metabolic capability. For in vivo experiments, 3-methylcholanthrene was injected intraperitoneally and the rats were killed 60 min later. Two additional rats were provided with biliary cannulae to divert bile from the pancreatobiliary ducts. Frozen sections, 16 mu in thickness, were prepared and examined under the fluorescence microscope. The sections revealed that fluorescence was concentrated in the epithelium and lumen of ducts. Animals with diversion of bile from the pancreatobiliary ducts had similar intensity and distribution of fluorescence. The in vivo studies showed that ductal epithelium was exposed to greater concentrations of carcinogen than nonductal epithelium. These observations provide a link between epidemiological studies that show an increased incidence of pancreatic adenocarcinoma in populations exposed to environmental carcinogens and morphological studies that show the ductal cell to be the most likely cell of origin.