Histopathological changes in gills, olfactory organ, liver, and intrahepatic pancreas of two estuarine fishes, the tidewater silverside Menidia beryllina and the hog choker Trinectes maculatus, exposed to south Louisiana whole crude oil (WCO) and its water soluble fractions (WSF) were assessed.  Both fishes were exposed to 5 mg WCO/l, 100 mg WCO/l, 5% WSF, and 50% WSF. Tests using M. beryllina lasted 21 to 30 days, whereas experiments using T. maculatus continued for 38 to 60 days. Pathological changes in the silverside included epithelial hyperplasia and fusion of gill lamellae, separation of respiratory epithelium from underlying tissue, hyperplasia of sustentacular cells of olfactory lamellae, necrosis of both neurosensory and sustentacular epithelium, extensive vacuolation in hepatocytes, and atrophy and necrosis of intrahepatic exocrine pancreatic nodules. In T. maculatus, on the other hand, the hepatocytes of exposed individuals did not become vacuolated, the olfactory mucosa exhibited severe necrosis only in fish in high oil concentrations, and, except for gills exposed to 100 mg WCO/l, response by gills in the remaining tests categories was limited to slight hyperplasia.  The intrahepatic pancreas of T. maculatus exposed to all test concentration atrophied and became necrotic at the end of the experimental period.  Pathological changes in the pancreas were observed consistently in both fishes and served as the most specific pathological indicator of oil pollution.  Menidia beryllina became hyperactive and disoriented within a few minutes after administration of oil and refrained from eating for up to six hours after initial exposure to hydrocarbons. Trinectes maculatus exposed to hydrocarbons were observed feeding during the test period and exposed individuals exhibited hyperactivity after all exchanges of seawater and fresh oil.  Mortality appeared to correspond both to degree of pathological damage and exposure-concentrations.  Partial to complete recovery was observed in gills, olfactory organ, and pancreas of M. beryllina exposed to WCO or WSF for 20 days and maintained in oil-Tree seawater for 17 days.