The purpose of this study was to investigate neonatal brain energy metabolism, acid, and lactate homeostasis in the period immediately following partial ischemia. Changes in brain buffering capacity were quantified by measuring mean intracellular brain pH, calculated from the chemical shift of Pi, in response to identical episodes of hypercarbia before and after ischemia. In addition, the relationship between brain buffer base deficit and intracellular pH was compared during and following ischemia. Thus, in vivo 31P and 1H nuclear magnetic resonance spectra were obtained from the brains of seven newborn piglets exposed to sequential episodes of hypercarbia, partial ischemia, and a second episode of hypercarbia in the postischemic recovery period. For the first episode of hypercarbia, brain buffering was similar to values reported for adult animals of other species (percentage pH regulation = 54 +/- 16%). During ischemia, the brain base deficit per unit change in pH was -19 +/- 5 mM/pH unit, which is similar to values reported for adult rats. By 20-35 min postischemia, brain acidosis partly resolved in spite of a net increase in lactate concentration. Therefore, the consumption of lactate could not explain acid homeostasis in the first 35 min following ischemia. We conclude that H+/HCO3- or other proton equivalent translocation mechanisms must be sufficiently developed in piglet brain to support acid regulation. This is surprising, because a substantial body of evidence implies these processes would be less active in immature brain. The second episode of hypercarbia, from 35 to 65 min postischemia, resulted in a smaller decrease in brain pH compared with the first episode, a result indicating an increase in brain buffering capacity (percentage pH regulation = 79 +/- 29%). This was associated with a parallel decrease in brain lactate content, and therefore acid regulation could be attributed to either continued ion translocation or the consumption of lactate. A mild decrease in brain pH and content of energy metabolites was observed, a finding suggesting that the metabolic consequences of severe postischemic hypercarbia are neither particularly dangerous or beneficial.