This study evaluated the potential mutagenicity of octamethylcyclosiloxane fluid using the dominant lethal assay. Four groups of 15 male Sprague Dawley rats were administered 0, 100, 500, or 1000 mg/kg/day by gavage 5 days/week for 8 weeks. In addition, a group of 15 males was similarly administered 0.05 mg/kg/day triethylenemelamine, the positive control. The animals were weighed weekly and observed for evidence of overt toxicity. After dosing ceased, treated and untreated males were mated with virgin, untreated Sprague Dawley female rats. The females were sacrificed on day 14 of pregnancy and the ovaries and uteri examined during necropsy. Number of corpora lutea, live implantations, and dead implantations were recorded. No treatment-related deaths occurred, and no behavioral changes were reported among the treated males. No significant effects on the fertility index was noted for treated males compared to control; a significant decrease in fertility was observed in the positive control group. No changes in body weight, fertility, number of implantations, or number of dead implants was observed in dams mated to treated males, compared to negative controls. No evidence of chromosomal damage in germinal tissue was found in the dimethyl cyclosiloxane fluid treated animals. As expected, the positive control substance produced evidence of chromosomal damage. A preliminary test failed to determine the maximum tolerated dose (MTD), therefore a MTD of 1 g/kg/day was assumed according to O.E.C.D. guidelines, since that was the highest dose tested.