Studies in patients undergoing peritoneal laparoscopy, in which carbon dioxide was used as insufflating gas, showed that artificial hyperventilation was an effective means of preventing serious hypercapnia. The hypercapnia caused a rapid hyperkalaemia and there was a linear relationship between the two. Reversal of the hypercapnia caused the serum potassium to fall towards normal but the speed of this fall did not match the speed of reduction in Paco2. It is suggested that the changes in serum potassium following changes in Paco2 may have some clinical relevance in potassium-depicted patients.