1. The assumption that the negative extracellular population spikes recorded from the pyramidal or granular layers in the hippocampal formation in response to appropriate afferent volleys is due to the sum of individual unitary discharges was tested by recording unit activity and population spike with microelectrodes, using normal and ultrashort amplifier time constants. 2. Unit spikes were correlated in time with the population spike. 3. The size of the population spike was altered by varying the stimulus strength, or by using a conditioning-test technique. In either case the number of units recorded followed the change in the size of the population spike. For very large population spikes, the technique failed since unit spikes could then no longer be clearly distinguished. 4. The theoretically expected shape of the population spike as a summation of individual unit discharges was derived, and discussed in terms of the observed results. 5. We conclude that over a wide range the height of the population spike is an increasing function of the number of discharging cells and can thus be used as a measure of the extent to which an afferent volley discharges a cell population.