In the Henderson-Hasselbalch equation, the apparent first dissociation constant for carbonic acid in plasma, pK1, is 6.10 ± 0.01 (±SD) in healthy adults. In contrast, values for pK1in sick adults and in sick infants and children have been reported to vary widely. Because of the far reaching implications of these findings, we repeated the measurements in 19 newborns in a neonatal intensive care unit. Two measurements were made in each infant, one while the infant was acutely ill and another after recovery. We found that neither the mean value nor the range of pK3values was affected by the infants' clinical status. The values during the acute phase of the hospitalization (range, 6.01-6.12; mean ± SD, 6.08 ± 0.03) did not differ from those after recovery (6.02-6.17; 6.08 ± 0.04). A second study was performed in order to see if the wide range of pK1values seen in other studies might be the result of an unstable state accompanying acute changes in acid-base status similar to those that might be encountered in clinical situations. However, data in seven lambs showed no significant difference when pK1before an acute alteration in acid-base status (6.10 ± 0.04) was compared with that 10 min after (6.09 ± 0.03). In newborn intensive care units, nomograms are used to calculate total CO2from pH and Pco2assuming a pK1=6.10. Our data support this practice.