Adult and juvenile Cyprinodon variegatus were exposed to relatively high sub-lethal concentrations of the potent carcinogenic agent, N-nitrosodiethylamine (DEN). Fish were exposed for several weeks under static conditions to initial DEN concentrations of 55 mg/l, dropping to 30 mg/l at exposure's end. Fish were then transferred to clean flow-through water for holding, observation, and characterization of lesion development. Progression of the hepatic effects of DEN is being followed by periodic sampling for histochemistry, histology, and electron microscopy. The earliest damage in the liver was assessed and subsequent progression of lesions is being monitored.