Plasma Somatomedin activity(SM) and cortisol were studied in 8 premature, 5 small for date and 6 at term newborns during the 3 first days of life. SM was measured by porcine cartilage bioassay, Cortisol by radioimmunoassay. SM at first day was detectable in 3 premature, 1 small for date and 3 at term newborns. SM increased at second day and it felt at 3th day in premature and small for date infants. All at term newborns porsued the SM increase in the 3th day too. The subjects with not detectable SM had very high circulating cortisol and those with low circulating cortisol showed always detectable SM levels. Small for date and premature plasmas showed an inhibitory effect on plasma standard, removed by heating at 60°C for 20 minutes. The high cortisol levels anc in addition a termolable substance(only in premature and small for date newborns) may explain the SM behaviour in newborns at the first days of life.