The present study investigates the developmental pattern of serum cortisol secretion in sick fullterm and preterm neonates in comparison to that of normal babies over the first 30 days of life. Four groups of babies (15 in each group) were studied sequentially during the first 4 weeks of life. Serial venous blood samples were drawn at 08.00,14.00, 20.00 and 02.00 h from each baby. The higher cortisol levels (p < 0.001) observed in sick preterm and full-term neonates, when compared to their respective controls, suggest an appropriate response to stress. Fullterm and preterm babies with no problem were found to have a free running rhythm in serum cortisol levels during the first 4 weeks of life.