AbstractThe acute toxicity of three materials derived from Athabasca Oil Sands—(1) bitumen plus naphtha, (2) untreated naphtha (0–250°C) and (3) synthetic crude oil (0–500°C)— was assessed in a battery of tests. In acute oral studies, all three test materials exhibited a low order of toxicity (LD50>5.0 g kg−1). The acute dermalLD50was also low (>3 g kg−1) for each test material. All three materials were judged to be ‘slight’ ocular irritants. Acute inhalation studies (6‐h exposures at the maximum attainable concentrations) produced varied responses. Bitumen plus naphtha administered at a concentration of 1.46 mg l−1did not cause mortality in exposed rats or mice. Lung discoloration was the only necropsy finding of note. Untreated naphtha administered at a concentration of 10.6 mg l−1was lethal to essentially all of the mice;but only two rats died. Necropsy findings included elevated weights in the liver and kidneys of the exposed mice, elevated lung weights in male rats and elevated liver weights in female rats. Synthetic crude oil administered at a concentration of (4 mg l−1) was lethal to 5/10 mice, but none of the rats (0/10) died. Severe hair loss was noted in the surviving mice, and slight alopecia was also observed in rats. Both species exhibited elevated liver weight, and elevated lung weight