24 hrs after acute carbon monoxide or m-dinitrobenzene poisoning (about 60% CO-Hb or Met-Hb) the sciatic motor conduction velocity of rats is significantly reduced by 33% and 16% respectively. Whereas 48 hrs after methemoglobinemia the nerve conduction is normal, a retardation is detectable even 4 weeks following carboxyhemoglobinemia. Also, 4 weeks after single carbon monoxide intoxication producing a mean CO-Hb content of 19%, the motor conduction velocity is significantly decreased. This effect may be useful for detection of carbon monoxide intoxication after elimination of the poison. The results show that there are carbon monoxide effects independent of blockade of hemoglobin.