A petroleum-derived hydrocarbon mixture, API No. 83-06, was evaluated for its acute inhalation toxicity. Five young adult rats of each sex were exposed to a target vapor concentration of 5.0 milligrams/liter (mg/L) for 4 hours. Vapors were generated by a countercurrent distillation process and administered in the breathing air of the animals utilizing dynamic whole-body exposure methods. During pre-exposure atmosphere characterization tests, API No. 83-06, when generated using the method produced an atmosphere that was entirely of vapors. Analysis of vapor concentrations was performed by continuously measuring total hydrocarbon content and yielded a time weighted average of 5.04 & 0.900 ag/L for API material. No animals died or exhibitea clinical signs during the course of the exposures or during the 14 day observation period. Individual and mean body weights were not affected by exposure to the test material. Histopathological examination of lung tissues yielded minimal to moderate pulmonary findings. The possibility that these could be attributed to the exposure could not be ruled out.