Recent research shows that in adult subjects GH follows a circadian rhythm with low daytime levels and high night-time values related to the beginning of sleep (1,4). This rhythmical pattern of GH serum levels is present also during childhood (8). In children, however, spontaneous GH rises occur during the day with a frequency and a greater elevation than in adult subjects (3,8). As GH release related to sleepwake cycles is not evident in newborns (5), the purpose of the present communication is to study at which age neuroendocrine correlations between sleep-wake cycles and GH secretion take place and, therefore, GH rhythmicity develops. We studied 41 infants, whose age ranged from 1 week to 1 year. All of them were born at term of pregnancy and their mothers had no symptoms of endocrine or metabolic disease. The subjects were divided into 4 groups: a) 9 newborns aged 1 week - 1 month, b) 14 infants aged 1 – 3 months, c) 10 infants aged 3 – 6 months, d) 8 infants aged 6–12 months. The babies were hospitalized for at least two weeks before study. In all these subjects plasma GH samples were obtained by venipuncture following at least 30 min of waking and 30 – 45 min after the beginning of sleep. GH was determined by a double antibody radioimmunoassay using 125I-GH,as previously described (7). Results are expressed in terms of Wilhelmi standard preparation of HGH (1 mg = 2 IU)