Chronic toxicity was evaluated in hamsters (53-59/sex/dose group) exposed to methyl methacrylate vapor at concentrations of 0, 25, 100, or 400 ppm for 6 hours/day, 5 days/week for 18 months. Mortality rates, in the control, low-, mid-, and high-dose groups were 34.5, 43.4, 35.7, and 60.0% (males) and 72.7, 64.9, 62.5 and 68.8% (females), respectively. Treatment did not affect body weight gain, hematocrit level or erythrocyte and leukocyte counts. Hemoglobin concentration was decreased in high-dose males sampled at 79 weeks, but was normal in all other groups. Numerous gross abnormalities were seen in control and treated rats,but no statistical analysis of these results was reported.