Naphthenic acids are a diverse group of saturated mono- and polycyclic carboxylic acids that are natural constituents of petroleum. These compounds are a major contributor to petroleum's acidic nature and can account for as much as 4% of crude petroleum by weight. At the Athabasca oil sands (AOS) located in northeastern Alberta, Canada, naphthenic acids have received considerable attention. The AOS represent the world's largest, single petroleum deposit, where the petroleum is in the form of bitumen. Extracting bitumen from AOS is a complex process, requiring the mixing of oil sands with hot,alkaline (pH = 8.0) water to separate the bitumen from sand and other waste products. This process produces an immense volume of aqueous tailings, about 7.5 m 3 for each m3 of synthetic crude petroleum produced. Nearly one billion cubic meters of aqueous tailings will have amassed in large holding ponds near the mine sites by 2025, and will be incorporated into the ecosystem under wet and dry landscape reclamation strategies. Another consequence of the extraction process is that naphthenic acids (pKa ≅ 5) become solubilized and concentrated (90--110 mg/L) in aqueous tailings. Numerous studies have investigated the aquatic toxicity of naphthenic acids, demonstrating them to be highly toxic to invertebrates and fish at concentrations well below those found in AOS tailings. In contrast, information about the mammalian toxicity of naphthenic acids is limited, particularly the effects of repeated, oral exposure. In the current research, naphthenic acids were isolated from tailings obtained from Mildred Lake settling basin, the main tailings pond of Syncrude Canada Ltd, and used in mammalian toxicity testing. An acidification/solvent extraction procedure was used, followed by ultrafiltration to isolate the naphthenic acids. These were chemically analysed, revealing a highly heterogenous mixture of acyclic and 1-, 2-, 3-, and 4-ringed compounds. Acute testing using Wistar rats demonstrated significant (P < 0.05) behavioural and histopathological effects in both sexes at a single dosage of 300 mg/kg body weight of napthenic acids. This dosage is 50 times higher than the estimated worst-case, single day environmental exposure through drinking water for small mammals in the wild. Effects included temporary suppression of appetite, and pericholangitis, a biliary inflammatory response. Subchronic dosing, involving administration of naphthenic acids to females over a 90 d period, indicated that 60 mg/kg/d was sufficient to elicit significant (P < 0.05), toxic effects.