The relationship between the spontaneous unit activity in the raphe dorsalis (RD), and the sleep-wakefulness cycles, was analyzed in the cat from birth to 40 days of age. Electrodes for polygraphic sleep monitoring were implanted under anesthesia, and unit recordings were obtained from bundles of microwires positioned in the RD area in kittens of different ages. Attention was paid only to units with slow firing in wakefulness (W) (1-6 spikes/s), and two types of discharge patterns during this state were obtained: a 'regular' type, whose discharge in W had the same characteristics of regularity as those described for the adult under the same conditions, was always found inside the RD. An 'irregular' type was always found in sites outside the RD. Injections of different doses of 5-methoxy-N,N-dimethyltryptamine (5-MeODMT i.m.) induced a transient decrease in the firing rate of the regular type of cells, and no change for the units of the irregular type, suggesting that the regular neurons were of serotoninergic nature. Whereas the cells of the irregular type exhibited an increase of discharge frequency in active or paradoxical sleep (AS-PS), those which fired in a clock-like manner during W exhibited a rate of discharge which progressively decreased in quiet or slow wave sleep (QS-SWS) and even more in AS-PS. Such a pattern was qualitatively close to the adult one at all ages, but the discharge rate in AS was significantly higher during the first and second weeks of life than later on. The observation that these serotoninergic neurons exhibited at birth an adult-like pattern of discharge during W, indicates that during ontogenesis there was no direct relationship between the RD activity and the behavioral output. It is proposed that the RD neuronal discharge would be largely under genetic influences, and that the maturation of sleep regulations at the brainstem and mesencephalic levels is achieved only after the second week of postnatal age.