Electrolytic lesions of the anterior raphe nuclei in the 4-6 day puppy rat were performed stereotaxically. The subsequent development of the sleep parameters in these animals was followed up to the second month of life. No difference could be found between the lesioned and control rats, neither in the qualitative nor in the quantitative characteristics of slow wave sleep (SWS) and paradoxical sleep (PS). However, when performing identical lesions on 3-5 week-old animals, a relative insomnia was obtained, more severe for PS than for SWS. The correlation of the neurophysiological data with the biochemical analysis of monoamines in the forebrain revealed that the serotoninergic (5-HT) system of the anterior raphe in the early lesioned animals was almost totally destroyed, with no recovery up to 9 months of age. Our results indicate that the anterior raphe nuclei are not functional during the first week of age, in regard to sleep control, whereas they do play an important role in sleep regulation after 3 weeks of age. Furthermore, some compensatory mechanisms which develop in the early lesioned animal as a consequence of the lesion, could explain the exhibition of normal sleep ontogenesis. The possible nature of these mechanisms is discussed in light of the monoaminergic theory of sleep control.