LA infusions have been used to examine the effects of metabolic acidemia on CBF, but LA may alter other variables important for CBF regulation. To compare effects of LA with differing pH but similar changes in blood gases, osmolality and infused volume, 14 acutely instrumented, spontaneously breathing piglets were studied by infusing LA with either 4M NaCl (LAI, n=7) or buffered with 5.5M NaOH (LA2, n=7). Blood flow (microspheres), pH, blood gases, and plasma osmolality and LA concentration were measured after a 30 min control (C), a 30 min LA infusion (I), and 15 (I+15min) and 90min (I+90min) after infusion completion. At C, pH was 7.49±.01 (X±SE) for LA1 and 7.48±.01 for LA2. Different values of pH* occurred for LA1 and LA2 at I, I+15 and I+90min; 7.09±.03, 7.35±.02, 7.46±.02 versus 7.58±.03, 7.61±.01, 7.57±.03, respectively. PaCO2 was similar between LA1 and LA2 during the study. In both LA1 and LA2 osmolality rose 15% during I and persisted throughout the study. Hyperlactatemia occurred with LA1 and LA2 and was greatest during I, and gradually decreased to baseline at I±90min. For LAI piglets, CBF (ml/min·100g) rose from 136±15 to 202±26* with I and remained elevated at 209±25 and 217±28 at I+15 and I+90 min. Similarly for LA2 piglets, CBF rose from 129±25 to 206±31* with I and was 173±17 and 169±21 at I+15 and I+90min. Therefore, lactic acid infusions increase CBF, but metabolic acidemia is not responsible for the observed changes. (*p<.01)