Dimethyl methyl phosphonate (DMMP) has been considered for use by the U.S. Armed Forces as a nerve gas simulant in a variety of experimental situations to simulate the physical properties of nerve gases, but not the neurotoxic properties. Dimethyl methyl phosphonate is also used as a flame retardant for urethane foams and polyester resins. This study was conducted to determine the reproductive toxicity of DMMP after subchronic dosing. DMMP was administered to male Fischer 344 rats by gavage 5 days/week for 90 days at dosages of 0, 250, 500, 1000, and 2000 mg/kg, and all animals survived this dosing schedule. At Day 84, the rats were mated to untreated female Fischer 344 rats. There was a dose-related decrease in sperm count, sperm motility, and the male fertility index. The male fertility index was 70, 75, 60, 40, and 0% in the 0, 250, 500, 1000, and 2000 mg/kg dose groups. DMMP acted as a dominant lethal mutagen as demonstrated by an increase in the number of resorptions with increasing doses of the drug. The percentage of resorptions in the control group was 6.1% and increased to 14.9, 37.8, and 79.1% in the 250, 500, and 1000 mg/kg groups, respectively. The testes of the male rats were examined histologically to determine the relationship between reproductive function and pathologic abnormalities. DMMP altered reproductive function at all dose levels, while histologic abnormalities of the testis were seen only in the high-dose group. Changes in the testes of the high-dose animals were characterized by lack of spermatogenesis or by degeneration, vacuolization, and necrosis of cells in the spermatogenic tubules. Histopathologic abnormalities of the kidney were seen in some animals from each of the dosed groups and microscopic changes of the prostate were seen in some of the high-dose animals.