The early changes in the systolic blood pressures in six clinical groups of newborn infants have been studied. Readings of the sphygmomanometer used have been compared with "direct" measurements during cardiac catheterization, and the results are reported. In 36 normal mature infants the systolic pressure fell by a mean of 25 mm Hg from the initial value recorded within the first 5 minutes after delivery, to the lowest value, reached as a rule between 1 and 4 hours after delivery. In 18 clinically asphyxiated mature infants, the mean fall (38.5 mm Hg) was significantlygreater because the initial level was significantly higher (by 15.1 mm Hg) than the comparable initial level in the infants who breathed promptly. Three episodes of apnea during anesthesia have also been observed in two infants aged less than 48 hours, and on each occasion there has been an abrupt rise in the systolic pressure, with or without a corresponding bradycardia. In a group of premature infants studied from within the first half-hour after delivery, the seven of them diagnosed as hyaline membrane disease had a significant hypotension between 5 minutes and 4 hours after birth, as compared with the lowest systolic pressures found at corresponding times in the 10 infants who remained well. Among infants of diabetic mothers, a similar relative hypotension was observed in the four severe cases of hyaline membrane disease as compared with blood pressures of the 11 who remained well and the three who were mildly affected. A group of 13 mature infants delivered by elective cesarean section, for indications other than maternal illness of any kind, showed no significant differences in the course of their lowest systolic pressures as compared with the group of mature infants delivered from below. Neither the cause nor the significance of the hypotension seen in hyaline membrane disease is yet sufficiently clear to allow recommendation as to treatment.