Despite an increasing availability of renin assays, there is little information in the literature reporting normal values of plasma renin activity in infants and young children. The diurnal variation or responses to the plasma renin activity of the various physiologic changes I children were little reported. The present study is to measure the normal values of plasma renin activity, diurnal variations and the changes following the assumption of upright posture. Fourty infants and children from 1 day to 8 year old were subjected to this experiment. The first samples were obtained by venepuncture at 7 a.m., second and third samples at 11 a.m. and 4 p.m. respectively from 8 newborns, 7 infants and 10 young children. The plasma renin activity measured by the radioimmunoassay method. The levels were highest in the newborn, progressive decline with increasing age. No notable diurnal variations were observed in the newborn, however, the serial plasma renin activity values of 1 year old infants showed almost the constant diurnal rhythms similar to that of the adults. The plasma renin activity levels were highest in early morning hours and gradually decreased by afternoon. Kept on at the upright posture from early in the morning on, the plasma renin activity of children from 6 to 8 year old increased at 11 a.m., however, all subjects, though in continued upright posture, showed the decrease of the plasma renin activity in the afternoon.