Polydimethylsiloxane fluids, at viscosities of 35, 350, and 1000 cs., were evaluated to determine their subchronic oral toxicity. Five groups of 100 male rats each received 0 (2 control groups) or, 10% polydimethylsiloxane fluids of each viscosity in their feed for 90 days. The animals were observed twice daily for signs of toxicity, appearance, behavior, and mortality. Body weights were recorded at the initiation of the study and weekly thereafter, while food consumption was determined weekly. Hematological parameters were measured prior to the termination of the study. All surviving animals were sacrificed and examined by both gross necropsy and histopathological examination. No statistically significant differences between treated and the control rats were reported in any endpoint examined with any of the three viscosities of the test materials.