In order to test the relationship between changes in plasma potassium concentration and pH changes of respiratory origin, we produced hypercapnia (mean PaCO2 71 mmHg = 9.5 kPa) in a group of 17 patients and hypocapnia (mean PaCO2 21 mmHg = 2.8 kPa) in another 20 patients during neurolept analgesia and intraabdominal operations. A control group of 19 patients was studied under normocapnia but otherwise identical conditions. During hypercapnia, serum potassium rose, deltaK/deltapH amounting to -0.82, -1.05 and -1.34 after 30, 60 and 90 min, respectively. During hypocapnia, serum potassium decreased, deltaK/deltapH being a little more negative than during hypercapnia (mean values -1.62, -2.44 and -1.60). Red cell potassium concentration decreased in all three groups to a similar extent. Blood lactate levels during hypercapnia decreased to 75% of control and during hypocapnia rose to a maximum of 186% of control. In order to obtain reasonable values for base excess in primarily respiratory acid-base disorders, it is necessary to use nomograms based on in vivo ECF-CO2-titration curves. With this premise, hypercapnia or hypocapnia in our patients was not associated with significant changes in base excess.