Single subcutaneous injections of rats with methyl methacrylate, 0.001 cc/g bw, caused mild depression, while 0.007 cc/g led to irregular respiration, asphyxia, coma and death. Doses of 0.007 cc/g led to blood in the urine. Cats subcutaneously injected with doses of 0.0025 cc/g or above showed dose-related depression, irregular respiration, asphyxia, coma and death, accompanied by blood in the urine. Toxicity to rabbits treated similarly was less pronounced than in cats, except for severe paralysis that affected all limbs in the 2 rabbits. Porphyrin extracted from urine of treated cats and injected into rats and mice appeared to sensitize the animals to sunlight. The fatal oral dose for 90% of rats administered single doses of methyl methacrylate by gavage was 0.009 to 0.01 cc/g, while 2 doses of 3 cc/g was lethal to cats. Repeated oral doses to rats of 0.002 cc/g every other day for 70 days caused reduced weight gain, while 0.003 cc/g led to death and blood in the urine. Dermal application of 1 cc test compound/day for 10 weeks led to edema in all treated rats, while a repeat exposure on 'hot summer days' caused only temporary local irritation. Single 8-hour inhalation exposure of rats to 30 mg/L of test compound killed approximately 70%, while 11.2 mg/L caused only slight irritation of the upper respiratory tract. Rats and mice exposed to 4.95 mg/L for 6 hours/day for 18 days by inhalation showed no toxic signs and only occasional blood in the urine. Rats and mice exposed via inhalation in constant darkness suffered less mortality than those exposed for 2 to 3 hours of direct sunlight/day. Continuous inhalation exposure of rats (3.5 mg/L for 22 hours), cats and rabbits (15 mg/L for 5 days) was fatal to 1 cat, and led to blood in the urine of rats and rabbits, but not the cats. Subcutaneous injection of dogs and rabbits to determine whether methyl methacrylate gives rise to increased excretion of oxalic acid gave negative results in both species.