Growth hormone has been estimated in blood sampled continuously in periods each lasting 30 min during the first 3–4 h of pentobarbitone-induced sleep in 69 children. With only two half-hour samples, almost the same information was obtained as with the estimation of growth hormone in all samples. In this way 95% of normally growing children showed growth hormone levels of 5 µU/ml or more. Children with growth retardation of unknown cause and overweight children showed on the average lower growth hormone levels, not rarely even below 5 µU/ml. Pituitary dwarfs all had maximum growth hormone levels of 3 µU/ml or less. Growth hormone levels during sleep may be normal in children who show negative results on provocation, while subnormal growth hormone levels during sleep have been encountered in some children with retarded growth who had a normal response upon provocation.