By simultaneously inserting three extracellular microelectrodes, separated by 1.0 mm from each other, into the cat's midbrain reticular formation (MRF), temporal cross-correlation of firing of neuron pairs was measured to investigate the mode of interaction among the MRF neurons during different states of sleep and wakefulness. None of 97 neurons pairs studied gave clear-cut cross-correlograms suggesting cascade connection between two neurons. However, 29 neuron pairs showed weakly synchronized firing which occurred periodically with a mean interval of 1.23 s. This interval was different from the respiratory cycle. The rhythmic synchronization appeared most obviously during slow wave sleep. The synchronized firing was encountered more often in the pairs of adjacent neurons picked up with a single electrode and in the pairs picked up with separate electrodes positioned along the mediolateral axis, when compared with the neuron pairs located along the rostrocaudal axis. About 80% of the neurons (37 our of 46) which showed periodically synchronized discharge had no corresponding periodicity in their auto-correlograms. Simple synaptic linkage conceivable for the observed periodic synchronization of a neuron pair would be shared inhibition with the common inhibitory source activated periodically. Some neurons in the dorsal raphe nucleus and in the solitary tract nucleus were also examined for the cross-correlation with the MRF neurons. However, no positive relationship was found.