The heavy oils produced from the Alberta oil sands contain cyclic organic compounds together with sulphur and nitrogen. Upon thermal treatment they have the potential to form carcinogenic, mutagenic, and toxic compounds. Recovery of oil by in-situ combustion processes, such as the Combination of Forward Combustion and Waterflood (COFCAW) process, may result in the formation of such biologically active compounds with generation dependent upon operating conditions. The objective of the present research program is to evaluate produced oils and accompanying waters from in-situ combustion processes for possible biological activity. This evaluation is based on biological testing, using the Ames test for mutagenicity and the Microtox test for toxicity, and on the chemical analysis of oils and waters for the presence of known carcinogens. For comparison, oils produced by the less thermally rigorous steam injection process, as well as naturally occurring bitumen, and synthetic crude oil and other oils produced from the Athabasca and Peace River oil sands were examined. Analysis of oils produced by in-situ combustion showed the presence of many carcinogenic and mutagenic compounds, among them the well known carcinogen benzo(a)pyrene (BaP). The BaP content of a mixture of bitumen, cracked oil, and diesel fuel produced by the COFCAW process from the Gregoire Lake pilot project contained 14 µg/g BaP. This compares with 1.5 µg/g in unaltered bitumen and 1 to 3 µg/g in most crude oils. Samples obtained from the Suffield Heavy Oil project which had not undergone rigorous thermal treatment contained from 1.5 to 7.5 µg/g benzo(a)pyrene and emulsion produced by steam injection from the Peace River Pilot project. operated by Shell Canada Resources Limited, contained 2.7 µg/g. An oil produced by dry retorting of the Athabasca oil sands contained 16 µg/g BaP. Oils produced from combustion tube experiments with Athabasca oil sand had a similar BaP content, 2.6 and 4.2 µg/g. Some tars and pitches, especially coal tars, may contain 10 to 100 times more BaP than crude petroleum. Refinery residuals, tars, and oils from Sarnia were found to contain 150 to 1050 µg/g. Benzo(a)pyrene is the best known of the carcinogens found in petroleum, but many other known or suspected carcinogens were found in greater quantities than BaP in the oils produced by in-situ combustion and dry retorting. The assessment of any carcinogenic hazard associated with petroleum is difficult. Animal tests are expensive and time consuming; therefore, short term bio-assays for mutagenic properties such as the Ames test, together with chemical analysis, are employed. Positive results in the Ames test are not an absolute indicator of carcinogenic potential. Mutagenicity does not in all cases imply carcinogenicity; however, those polycyclic aromatic hydrocarbons which are carcinogenic are also mutagenic in the Ames test when appropriate enzymes are included. Mutagenic activity was found with the Ames test in oils produced from the Gregoire Lake. Suffield, and Peace River in-situ pilot projects and in vacuum gas oil and pitch from the Peace River diluent recovery unit. The mutagenicity was less than predicted from the amount of carcinogenic aromatic compounds found by chemical analysis. The complex mixture of hydrocarbons which comprises these oils suppressed the activity of the carcinogens in the Ames test. Thus the Ames test was found to be an indicator of mutagenic activity but not a quantitative method for assessing the relative mutagenicity of oils. Synthetic crude oil produced from Athabasca bitumen displayed some mutagenic activity but, despite the presence of BaP, the bitumen itself did not. Waters produced during in-situ recovery of oil by both steam stimulation and combustion processes were toxic to aquatic organisms as determined by the Microtox bioluminescence assay. EC 50 values, the effective concentration of toxicant causing a 50% decrease in the light output of a photoluminescent bacteria, ranged from 0.30 to 11. The toxicity was caused partly by volatile organic compounds, primarily alkyl substituted benzenes, and partly by extractable organic compounds including phenols, organic acids, and hydrocarbons with no single class of compounds solely responsible for the observed toxicity. Wastewaters from the dry retorting process were more toxic than waters produced by in-situ combustion and contained many aromatic hydrocarbons and nitrogen compounds known to be biologically active. The chemical analyses and limited biological testing carried out in this study detected no strong mutagenic or carcinogenic hazard associated with in-situ recovery of heavy oil by combustion and steam injection. The relative hazard is probably marginally greater than that associated with production of conventional light crude oils but far less than might be expected from coal liquefaction processes or from disposal of refinery residuals. The hazard associated with dry retorting is greater than that from in-situ recovery methods and care should be taken in the handling of both products and wastewaters from this process.