AbstractThe latency of onset and the duration of sleep induced by pentobarbital was studied in rats from 9 to 35 days of age, as well as in adult rats. Sleep latency was prolonged and sleep duration shortened with increasing age and maturation. Electrocorticographic changes after pentobarbital administration were studied in rats of 5–45 days of age and in adult rats. In rats 9 days of age or less the electrocorticogram manifested only depression of activity, whereas from the 12th day onward all characteristic barbiturate‐induced phenomena were registered: spindles, slow waves, and isolated spikes with suppression of background activity. Spindles were the most conspicuous of the pentobarbital‐induced phenomena and were the 1st to appear as a function of age, occurring only in the frontal areas at 12 days of age but in both frontal and occipital areas at 15 days of age. Frequency of the elements comprising spindles increased with from 2.5–3.5 c/sec in 12‐day‐old rats to 5–10 c/sec in adult animals. Barbiturate spindles could be used as a model for rhythmic thalamocortical phenomena even at early stages o