Antidromic activation of layer 6 cortico-geniculate cells from the lateral geniculate nucleus (l.g.n.) was used to study synaptic effects mediated by their intracortical axon collaterals. A specific stimulation procedure, earlier shown to suppress geniculo-cortical synaptic effects and to enhance synaptic effects from layer 6 cell collaterals was employed to differentiate between inputs from the two pathways. Single cells in different cortical layers were recorded both extra- and intracellularly with glass micro-electrodes. Antidromic activation of cortico-geniculate cells at 10-16 Hz induced massive, usually repetitive, spike discharges in almost all cells in layers 2-5. The shortest latency for this synaptic activation (3.4-7.5 ms) was found for simple cells in layer 4. Cells in layers 2, 3 and 5 responded with progressively longer latencies. Cortico-geniculate cells in layer 6 were atypical in that hardly any responded with synaptic discharges to the stimulation. Intracellular recordings from layer 4 cells revealed, besides a monosynaptic excitatory post-synaptic potential (e.p.s.p.) from geniculo-cortical fibres, a late e.p.s.p. with a latency of 3.0-4.2 ms. This e.p.s.p. could only be evoked from stimulation sites within the A laminae of the l.g.n. in retinotopic register with the recording site in the cortex. No corresponding potential was obtained by stimulation of the optic tract or the superior colliculus. The threshold intensity for the late e.p.s.p. in layer 4 cells was much higher than for the geniculo-cortical e.p.s.p.s from the same stimulation sites, indicating that the effect was mediated by thin fibres. The late e.p.s.p.s increased dramatically in size with repetitive stimulation of the l.g.n. at frequencies above 7 Hz, while the geniculo-cortical e.p.s.p.s remained unchanged. In all these properties, the late e.p.s.p.s resemble similar e.p.s.p.s evoked monosynaptically in principal cells of the l.g.n. by orthodromic activation of cortico-geniculate fibres. Large e.p.s.p.s were evoked also in complex cells of layers 2, 3 and 5 and in simple cells of layer 6 upon antidromic stimulation of cortico-geniculate fibres. The sample included many efferent neurones, identified as projection cells by antidromic activation from extracortical stimulation sites. The latency and behaviour of the e.p.s.p.s in these cells indicate that the effect was mediated indirectly via layer 4 cells. Presumably, the excitation travelled along an earlier identified chain of neurones, going from layer 4 to layers 2 and 3, from there to layer 5 and then to layer 6.(ABSTRACT TRUNCATED AT 400 WORDS)