Two crude oils, differing in viscosity (V) and nitrogen (N) and sulfur (S) content, were evaluated for systemic toxicity. In the Crude I (low V, low N, low S) study, the material was applied to the clipped backs of rats at dose levels of 0, 30, 125, and 500 mg/kg. In the Crude II (high V, high N, moderate S) study, the oil was applied similarly at the same dose levels. The crude oils were applied for 13 wk, 5 d/wk. Exposure sites were not occluded. Mean body weight gain (wk 1-14) was significantly reduced in male rats exposed to Crude II; body weight gain of all other animals was not adversely affected by treatment. An increase in absolute (A) and relative (R) liver weights and a decrease in A and R thymus weights were observed in male and female rats exposed to Crude II at 500 mg/kg; only liver weights (A and R) were adversely affected in male and female rats exposed to Crude I. In general, there was no consistent pattern of toxicity for serum chemistry endpoints; however, more parameters were adversely affected in Crude II-exposed female rats than in the other exposed groups. A consistent pattern of toxicity for hematology endpoints was observed among male rats exposed to Crude I and male and female rats exposed to Crude II. Parameters affected included: Crudes I and II, red blood cell count, hemoglobin, and hematocrit; Crude II, platelet count. Microscopic evaluation of tissues revealed the following treatment-related findings: Crude I, treated skin, thymus, and thyroid; Crude II, bone marrow, treated skin, thymus, and thyroid. The LOEL (lowest observable effect level) for skin irritation and systemic toxicity (based on marginal effects on the thyroid) for both crude oils was 30 mg/kg; effects were more numerous and more pronounced in animals exposed to Crude II. Systemic effects are probably related to concentrations of polycyclic aromatic compounds (PAC) found in crude oil.