The inhalation toxicity of 50:50 mixtures of n-butane-n-pentane or isobutene: isopentane was evaluated in two studied conducted simultaneously using common controls. Male and female Fisher 344 rats were exposed to the hydrocarbon mixtures at 4500 ppm and 1000 ppm for 6 hrs per day, 5 days per week for 13 weeks. Chamber atmospheres were monitored by Infrared spectroscopy and gas chromatography. Mean daily concentrations were within (plus or minus)10% of the target values for 99% of all exposure days. Minimal numbers of exposure-related toxic effects were observed in the rats throughout the study. Body weights of the isobutene/isopentane-exposed rats were unaffected relative to those of controls. Inhalation of the n-butane/n-pentane mixtures however produced statistically significant decreases in body weights by Test Weeks 3 and 4 for both sexes, with male rats recovering towards the end of the exposure period. Gross lesions observed at necropsies performed for subgroups of male rats after 4 exposure weeks, and for the remaining male and all female rats after completion of the 13-week period were considered to be spontaneous and unrelated to the exposures. Liver and kidney weights were unaffected by the exposures. The kidneys, submitted to Experimental Pathology Laboratory, Inc. for histopathologic examination, failed to reveal any evidence of hydrocarbon nephropathy in male or female rats from any treatment group.