Epinephrine lowers the serum potassium level through an effect on the β2-receptor. It is therefore difficult to evaluate whether depression of the ST level and the T wave on the electrocardiogram, during regional anaesthesia, is caused by a high plasma epinephrine level, a low serum potassium level, or both. For this purpose, we studied the relation between electrocardiogram and the plasma epinephrine and serum potassium concentrations during 20 intra-abdominal operations performed under combined epidural and general anaesthesia and in 18 subjects receiving intercostal nerve blockade. The results show that the hypokalaemic effect of epinephrine was reduced when the plasma epinephrine concentration exceeded 3 nmol L−1. A multiple regression analysis demonstrated that the ST level and the T-wave amplitude were most consistently affected by a low serum potassium level, while a high plasma epinephrine level correlated only to a depression in the T-wave amplitude. During surgery, only serum potassium correlated with these ECG variables.