Subchronic 90-day feeding studies were conducted in male and female Fischer-344 (F-344) rats on highly refined white mineral oils and waxes representative of those used for food applications. The goal was to help clarify the mixed results found in other toxicity studies with laboratory animals. Seven white oils and 5 waxes were fed at dietary doses of 20,000, 2,000, 200, and 20 ppm and compared with control groups on untreated diet; toxicity was assessed at 90 days and also after a reversal period of 28 days and/or 85 days. Higher molecular-sized hydrocarbons (microcrystalline waxes and the higher viscosity oils) were without biological effects. Paraffin waxes and low- to midviscosity oils produced biological effects that were inversely related to molecular weight, viscosity, and melting point; oil type and processing did not appear to be determinants. Biological effects were more pronounced in females than in males. Effects occurred mainly in the liver and mesenteric lymph nodes and included increased organ weights, microscopic inflammatory changes, and evidence for the presence of saturated mineral hydrocarbons in affected tissues. Inflammation of the cardiac mitral valve was also observed at high doses in rats treated with paraffin waxes. Further studies are required to elucidate the mechanism for the responses observed and the relevance of these inflammatory responses in the F-344 rat to other species, including humans.