A remarkable feature of motor cortical organization in higher mammals is that a brief electrical stimulus elicits in the pyramidal tract and corticospinal tract an unrelayed direct (D) wave followed by multiple indirect (I) waves at frequencies as high as 500-700 Hz. This review presents some conclusions regarding very high frequency synchronous activity in mammalian cortex: (1) Synchrony in repetitive I discharges is extraordinary in humans and monkeys, less in cats and still less in rats, being there represented by a delayed broad wave; such phylogenetic trends have important implications for the suitability of lower mammalian species for studies of high frequency cortical networks in the human brain; (2) The evidence from microstimulation at different cortical depths and pial cooling favors a vertically oriented chain of interneurons that centripetally excite corticospinal neurons as the basis for inter-I wave periodicity and synchrony; (3) Significantly, the I wave periodicity is conserved despite wide changes in stimulus parameters; (4) Synchronous high frequency activity similar to that of I waves can be recorded from other neocortical areas such as visual and somatosensory cortex; however, evidence is still lacking that the output neurons of these cortical regions have synchronized discharges comparable to I waves; (5) In limbic cortices, the frequency of synchronous neural activity is lower than that in motor cortex or related cortices and periodicity is not conserved with changes in stimulus parameters, indicating a lack of the neocortical interneuronal substrate in limbic cortex; (6) We propose that the very high frequency synchronous activity of motor cortical output reflects a computational function such as a "clock," quantizing times at which inputs would interact preferentially yielding synchronous output discharges. Such circuitry, if a general feature of neocortex, would facilitate rapid communication of significant computations between cortical regions.