The spatio-temporal distributions of excitatory synaptic ensemble activities in A17 and A18 of the visual cortex of the macaque monkey have been investigated. The synaptic activities were elicited by electrical stimulation of the primary efferents and were localized by applying the current source density analysis to the intracortically recorded field potentials. The principal results are as follows: 1. In A17, two groups of activity, evoked by fast and slow afferents, respectively, were distinguishable. 2. The fast afferents induced monosynaptic activity in layer IV C alpha and layer VI, disynaptic activity in layer IV C alpha and in the supragranular layers and trisynaptic activity in layer IV B. 3. The slow efferents induced monosynaptic activity in lower layers IV C beta and layer VI, disynaptic activity via strong connections in upper layer IV C beta, further disynaptic activity in layers III and IV B and trisynaptic activity in layers V A and II. 4. With the exception that the CSD data reveal more polysynaptic activity within layer IV, there is good agreement between the spatio-temporal distribution of synaptic activities and the cortical circuit diagrams proposed in anatomical studies. 5. In A18, activities from the slow and fast conducting afferent systems are revealed in layer IV, both most likely mediated by the monosynaptically activated target cells of A17. These activities are passed on to the supra-and infragranular layers. 6. In the lateral geniculate nucleus the safety factor of transmission is higher for activity conveyed by slow-than by fast-conducting retinal afferents. 7. The spatial distribution of monocularly evoked surface potentials failed to reveal the ocular dominance columns. 8. Comparison with the cat indicates that, with respect to the intracortical circuitry and LHN-transmission, there are more similarities between the fast-group activity in the monkey and the y-system in the cat and between the slow-group activity in the monkey and the x-system in the cat than vice versa.