Groups of Sprague-Dawley rats received sodium monofluoroacetate (Compound 1080) at 0.025, 0.075, and 0.25 mg/kg by oral gavage once daily for 90 days and were then euthanized. The control and 0.25 mg/kg/day groups included additional rats of each sex that were treated for 90 days, then maintained without treatment for a further 56-day recovery period. Microscopic changes were restricted to the testes and the heart, and were seen only in males dosed with 1080 at 0.25 mg/kg/day and included severe hypospermia in the epididymides, severe degeneration of the seminiferous tubules of the testes, and cardiomyopathy. Sperm evaluation indicated severe decreases in all three sperm parameters evaluated (concentration, % motile, and % abnormal) at 0.25 mg/kg/day. There were no microscopic changes or 1080-related effects on sperm parameters at 0.025 and 0.075 mg/kg/day. The no observable effects level (NOEL) for rats administered 1080 via oral gavage for 90 days was 0.075 mg/kg/day. The lowest observable effects level (LOEL) dose was 0.25 mg/kg/day. After dosing with the LOEL dose of 0.25 mg/kg/day, mean concentrations of 1080 in rat plasma were 0.26 micro g/ml at 1 h and 0.076 microg/ml at 12 h. Rat urine collected from the same animals contained 0.059 microg/ml.