In neocortical brain slices of the rat that were exposed to 50 microM picrotoxin, low-intensity stimuli evoked all-or-none epileptiform events that propagated across the slice with an average velocity of 0.07 m/s. Simultaneous recordings from pairs of electrodes, in which one was held in a constant position and the other was systematically advanced across the slice in small steps, revealed that propagation of the synchronous activity was saltatory rather than uniform. Analysis of the propagation pattern showed that local regions (< 1000 microns) of uniform velocity were separated by distinct borders. Within these regions, local propagation velocity was determined by the threshold for synchronous activation of still-smaller (< 200 microns) neuronal aggregates. Although the velocity was sensitive to physiological factors that affect the precise threshold for synchronization, the location of the borderlines between adjacent regions remained unchanged. We propose that these invariant borders reflect the details of local neuronal organization within the slice, and that the pattern of propagation of epileptiform discharge is a manifestation of the intrinsic organization of the neocortex when deprived of afferent input.