Proton magnetic resonance spectroscopy (1H MRS) performed in vivo on nine Sprague Dawley rats detected a threefold increase in whole brain lactate during intravenous 1 mol/L sodium lactate infusion. Significant increases in whole brain lactate were detected within 5 min after starting lactate infusion, progressively rose to a maximum level estimated at 3.2 +/- 1.5 mmol/L (all values +/- SD) immediately postinfusion, then decreased towards baseline levels during the next hr. Venous lactate concentration, increasing from 2.3 +/- 2.4 mmol/L to 43.0 +/- 8.0 mmol/L during the infusion, exhibited a steeper rise and then decreased more rapidly in comparison to changes in whole brain lactate. These data suggest MRS can be used in vivo to study acute changes in brain lactate associated with increasing blood lactate concentrations.