A study was conducted to evaluate the potential toxicity of hexamethyldisiloxane following inhalation exposure. Three groups of 5 male and 5 female Sprague-Dawley rats were exposed via inhalation to concentrations of 0, 499 ppm, or 1004 ppm hexamethyldisiloxane, for 6 hours per day, 5 days per week, for 2 weeks. The animals were observed daily for mortality or treatment-related toxicity, including respiratory, dermal, behavioral, nasal, and ocular effects and/or body weight changes. Following the final exposure, animals were sacrificed and gross pathological exams, including weighing of organs, as well as histopathological exams were conducted. All animals survived to terminal sacrifice. There were no significant differences in mean body weight in treated animals compared to controls. In the males only, there was a dose-related trend toward increased mean kidney weight. When expressed as kidney to body weight and kidney to brain weight ratio statistically significant increases were observed in the 1000 ppm males. These organ weight changes appeared to correspond with an increased severity of hyaline droplets in the proximal convoluted tubule, ie. hydrocarbon nephropathy, of the treated male rats. Hydrocarbon nephropathy is unique to male rats and is not observed in humans. No other significant toxicological effects were observed.