Mice showed no toxic signs after a single injection of o-sec-butylphenyl methylcarbamate (BPMC, 10 mg/kg) or 2-isopropoxyphenyl-N-methylcarbamate (propoxur, 2 mg/kg). Each dose of BPMC or propoxur caused an increase in acetylcholine content and a decrease in acetylcholinesterase activity in the forebrain of mice at 10 min, followed by an almost complete recovery in the content at 60 min. Spontaneous motor activity was depressed 10 min after and recovered 60 min after, injection of BPMC or propoxur. Neither rotarod performance nor rectal temperature showed any change after injection of BPMC or propoxur. Spontaneous motor activity may therefore be a simple method for assessing small changes in the cholinergic system.