Electrolytic lesions were used to destroy the anterior raphe nuclei and the loci caeruleus and subcaeruleus in kittens at varying stages of maturation. No lesion performed in the first week after birth led to changes in the characteristics of sleep in the 8 post-operative days. On the other hand, lesions performed in kittens aged 1-2 months had the same effect as is described as occurring in the adult animal. The structures which are responsible for states of alertness thus appear to start acting during the post-natal period.