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1. Extracellular records from one type of spontaneously active element in the cortex of rats anaesthetized with urethane have been analysed statistically.

2. Intensity functions reveal that the frequency of discharge of a unit tends to be modulated periodically, the period being 1-2 sec under fairly deep anaesthesia. When anaesthesia becomes lighter, long intervals occur rarely; the unitary discharge becomes less regular and the intensity function becomes flat. Variations in depth of anaesthesia are accompanied by changes in amplitude of the periodic modulation; the period of the modulation is not significantly altered.

3. Summed interval histograms indicate that the long intervals are separated by groups of short intervals in a significantly regular fashion. The number of short intervals in a group varies from one to about fifteen, according to the mean frequency of discharge of the unit.

4. Serial correlation coefficients reflect the observations of 2 and 3 above but indicate no additional serial correlation. The existence of long intervals obscures any serial patterning of short intervals during bursts.

5. Simultaneous records from two units show that the periods of activity in the two units are highly synchronized.

6. It is probable that all these features are due to a process which causes periodic synchronous excitability changes of all the spontaneously active elements of the type studied.