Mine depressurization water obtained from five wells on Lease 17 held by Syncrude Canada Limited, was examined for chemical composition and acute toxicity to two species of fish. In the first series of experiments, mine depressurization water was diluted with various proportions of water obtained from the Athabasca River, and trout-perch (Percopsis omiscomaycus) were exposed to these mixtures for up to 10 days. These experiments were performed in a mobile laboratory located in Fort McMurray. The 96-hour lethal concentrations (LC50's) ranged from 20% by volume (Well No.5) to 48% by volume (Well No.1). The 96-hour LCso's for the composite samples ranged from 35% by volume to 45% by volume. Similar studies were undertaken in the second series of experiments in Edmonton, using rainbow trout (Salmo gairdneri) with Edmonton City water as the diluent. Four of the five wells previously tested were studied, with resulting 96 hour LC50's of between 20% and 40% by volume for Well No.2, and 60% and 80% by volume for the other three wells. In addition, a study was performed on a composite of these four wells to determine the effect of storage time on toxicity. It was observed that toxicity decreased after 10 days storage (96-hour LC50's of between 40% and 60% volume to between 60% and 80% by volume) but then increased (96-hour LC50 of 15.2% by volume) after 20 days storage. Considerable variations in toxicity were found between wells and even water from a single well varied in toxicity depending on the time the sample was obtained and how long it had been stored. Variations in the chemical composition of the mine depressurization water were observed for such components as zinc, nickel, and iron between sample periods, as well as for concentrations of sodium, chloride, and other components from well to well.