The interaction between areas of the cerebral cortex of the cat have been studied by recording the electrical responses of one area following stimulation of another area by single and repetitive shocks. The cortex is stimulated at different depths, and records are taken from one electrode at the cortical surface and another in white matter, or from fractional leads, in the area responding.Axons from the stimulated point of one region may end in the second region by synaptic connections either on cell bodies or on cell dendrites. The paths ending on the apical dendrites of pyramidal cells are particularly dealt with here. The type of ending can be recognized by the form of the activity set up in the neurones responding.If axonal endings on cell bodies are involved, the sequence of events starts with one or more brief spikes (1 msec. in duration) characteristic of all-or-none activity in cell bodies and their axons. If the paths activated end on dendrites, a slower wave (15 msec. in duration) is the initial and may be the only response.Certain conclusions can be drawn from the experimental results. The properties of cell dendrities are different from those of cell bodies and their axons. The duration of response of dendrites is much longer; they have no true absolutely refractory period; they may support local activity at a stimulated region without propagation of impulses to their cell bodies, and then affect their cell bodies by altering the excitability of the latter to impulses reaching them directly. They can be maintained in persistent negativity by repetitive stimulation.Of relations so far studied, the afferent projection areas are activated chiefly by means of axonal paths ending on cell bodies, while many of the interconnections from cortex to cortex are made by axonal paths ending on dendrites.Abstract Teaser