To determine the pattern of unitary activity in secondary epileptic foci of Macaca mulatta when situated in association cortex, aluminium hydroxide cream was applied unilaterally to the inferotemporal cortex. When secondary abnormal discharges (transmitted or independent) were observed in the EEG, recordings were taken of mass and extracellular unit activity in both primary and secondary foci. All units situated in the cortex from which mass spike activity could be recorded changed their discharge frequency time-locked to the mass event: a decrease in firing frequency starting 20 to 50 ms after the peak of the mass spike and ending 100 to 300 ms later was invariably present; about half the units also increased their rate of discharge either before or after the decrease. No unit in cortex free of mass spike activity showed any change in firing frequency time-locked to the spikes from the opposite hemisphere. All changes in unitary firing lagged about 20 ms on the peaks of interictal mass spikes; but with ictal spikes an early increase in firing rate coincided with the spike peaks. The changes in firing time-locked to mass events were not associated with the type of spontaneous unitary activity. No difference was found in the behavior of units in primary or secondary foci, or in relation to the age or evolution of a focus.