The potential health effects of a raw shale oil were evaluated in a 90-day inhalation study in Fischer 344 rats. Groups of 15 male and 15 female rats were exposed 6 hr/day, 5 days/week for 13 weeks to aerosol concentrations of 0, 56, 120, or 492 mg/m3. In the high-dose group, 10 males and 7 females died prior to the termination of the study, most within the first 5 weeks of the experiment. A dose-dependent suppression in weight gain was seen in all of the shale oil-exposed groups. The failure to gain weight was associated with a variety of clinicopathologic abnormalities, including a dose-related decrease in red and white blood cells, with lowered plasma protein levels and increased serum alkaline phosphatase, and with total bilirubin levels in males. The exposure of the test animals to aerosolized raw shale oil was also associated with inflammatory and hyperplastic lesions in the lungs and upper respiratory tract, atrophy of the thymus and thymic-dependent portions of the peripheral lymphoid system, and bone marrow. These changes demonstrate that inhalation of raw shale oil aerosol can produce major organ toxicity similar to that found after exposure to other unrefined oil products.