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Systolic, diastolic, and mean aortic blood pressure measurements taken during the first 12 hours of life in 16 clinically stable, untransfused infants who weighed 610 to 980 gm at birth were analyzed. These infants were selected from 207 infants weighing <1,000 gm admitted to our hospital between 1965 and 1978. Selection criteria were pH ≥7.25, Paco2 <50 torr, Pao2 >50 torr, hematocrit >40%, inspired oxygen ≤40% at 6 hours of age. Blood pressures of appropriate for gestational age and small for gestational age infants of comparable weight were similar. From linear regressions of blood pressures on birth weight, average values and 95% confidence limits for two different birth weights were derived. For infants weighing 750 gm the mean aortic blood pressure, measured in torr, was 33 (range 24 to 42); systolic, 44 (range 34 to 54); diastolic, 24 (range 14 to 34). Mean blood pressures of infants weighing 1,000 gm were 34.5 (range 25 to 44); systolic, 49 (range 39 to 59); diastolic, 26 (range 16 to 36). These values are lower than those extrapolated from larger infants using a parabolic regression. Data from these 16 infants were combined with data from 45 larger infants to compute new nomograms for aortic blood pressures during the first 12 hours of life in infants weighing 610 to 4,220 gm. The relations between blood pressures and birth weights were best described by linear regressions. The lower limits of normal mean aortic blood pressure are 25 torr at 750 gm, 29 torr at 1,500 gm, and 37 torr at 3,000 gm; the lower limits of normal systolic blood pressure are 34 torr at 750 gm, 40 torr at 1,500 gm, and 51 torr at 3,000 gm. These extended nomograms should replace the parabolic regression of mean aortic blood pressure vs weight which may have inaccurately indicated hypotension in infants of very low birth weights.