Male and female rats were exposed to propoxur (2-isopropoxyphenyl-N-methyl-carbamate) aerosol, concentrations averaging 5.7, 18.7 and 31.7 mg/m3 air, for 6 hours daily on five days in a week over a period of 12 weeks. The effects were depressions of plasma by 20 to 30%, and of erythrocyte and brain cholinesterase activities which were caused by the highest air concentration of 31.7 mg/m3. On the basis of the physical, biochemical, phamacokinetic and metabolic behaviors of propoxur, a maximum allowable concentration of 2.5 mg/m3 air is suggested.