The properties of both the compound action potential (AP) and summating potential (SP) were studied in 18 normal human cochleas. The summating potentials were investigated with respect to the input-output behaviour and occurrence of positive and negative summating potentials. The compound action potential is discussed with respect to: a. the amplitude-intensity and latency-intensity behaviour, with special reference to the separation into low and high intensity parts; b. the action potential waveform and particularly the occurrence of a double-peaked N1 and the properties of both peaks; c. the adaptation and masking properties; d. the amplitude-latency and width-latency relations describing the internal synchronization for the compo nents of the compound action potential. A regression analysis was performed for the amplitude-latency scattergrams to provide adequate data reduction for clinical reference. Detailed properties of the two populations of neural units are given, and a morpho-logical substrate is proposed. Finally, a comparison is made between the results obtained in man and animal.