Abstract Patterns of spontaneous unitary activity were analysed in the ventro-lateral thalamic nucleus of unanaesthetized cats. Patterns of discharges were correlated with the electrocorticogram. A new logrithmic joint interval distribution has been introduced to study the serial order of intervals ranging over a large domain of time. During slow cortical waves, burst of high frequency discharges appeared, as well as under light chloralose anaesthesia, where they had a sustained rhythmic character. The intervals within the bursts lengthened. The mode of interspike interval distributions of the intervals composing the bursts ranged from 1.8 to 3.6 msec. During slow cortical waves there was no synchronization of the bursts from two cells recorded with the same microelectrode. During fast cortical waves the type of activity observed represents a random process with a slight dependence between successive intervals. The modes of the corresponding interspike interval distributions ranged from 4 to 22 msec. Under light chloralose anaesthesia no correlation was observed between interburst intervals and measurements describing bursts.