Abstract The behavior of different components (D, I 1 and I 2) of the pyramidal tract population response of the cat, evoked by surface stimulation of motor cortex, was examined during repetitive stimulation and cortical cooling. The D, I 1 and I 2 waves could be separated on the basis of their frequency following characteristics. The I 2 wave failed at rates above 50/sec, the I 1 wave at rates above 300/sec and the D wave at rates above 1000/sec. Both the D and I 1 waves could be potentiated by repetitive stimulation at threshold stimulating intensities. The I 2 wave was not potentiated during tetanic stimulation. A further differentiation between the behavior of the D and I 1 waves occurred during cortical cooling. The I 1 wave was reduced in amplitude, increased in latency and finally abolished as cortical temperatures were lowered from 36.5 to 22.0 C. The latency of the D wave was unaffected and its amplitude slightly augmented by a similar decrease in cortical temperature. It is concluded that while both the I 1 and I 2 waves arise out of synaptic excitation of pyramidal tract cells, different synaptic systems are involved. The I 1 wave deflection may originate from a monosynaptic of fast disynaptic connection to pyramidal cells, whereas the I 2 wave deflection probably involves a polysynaptic system.