Gastric acidosis as assessed by tonometry was applied to evaluate changes in splanchnic blood flow during exercise. In six healthy male oarsmen, we determined gastric acidosis in response to 30 min of maximal ergometer rowing. The gastric mucosa carbon dioxide tension was determined by equilibration of isotonic saline to the tonometer. Arterial bicarbonate (HCO3-), pH, arterial oxygen tension (PaO2), and saturation (SaO2) were obtained simultaneously, while pH (pHi) of the gastric mucosa was calculated using the Henderson-Hasselbach equation. During rowing PaO2 and SaO2 decreased to values of 73.7 mm Hg and 95.5%, respectively (P < 0.05). However, during the last minute of rowing the values were normalized with a hyperventilation reducing PaCO2 to 27.1 mm Hg (P < 0.05). Rowing decreased HCO3- from 25.8 (21.4-28.5) to 14.1 (11.6-17.4) mmol l-1, while the gastric carbon dioxide tension increased from 36.8 (24.1-63.9) to 61.7 (48.9-82.0) mm Hg (P < 0.05). Accordingly, pHi decreased from 7.25 (7.04-7.48) to 6.79 (6.67-6.85) (P < 0.05). Arterial pH also decreased (from 7.42 (7.41-7.44) to 7.29 (7.26-7.33) (P < 0.05)), with the enlarged difference between pH and pHi suggesting marked splanchnic hypoperfusion during rowing.