The effects of exposure to methane sulfonylfluoride at concentrations of 20 or 100 ppb for a total of 61 exposures of 7 hours each were studied in male and female rats. The control and exposed groups did not differ in appearance, behavior, weight gain or food consumption. There was no effect on plasma cholinesterase. RBC cholinesterase was significantly depressed in the high-level male and female exposure groups. There was no significant accumulation of fluoride in either the liver or blood at either exposure level.