BACKGROUND: The aim of this study was to compare the effect of propofol with succinylcholine and thiopentone with succinylcholine on serum potassium concentration during induction of general anesthesia. METHODS: Forty patients scheduled for elective surgery were allocated at random into two groups, one to receive propofol with succinylcholine or other to receive thiopentone with succinylcholine. We measured serum potassium concentration at preinjection and at 1, 5 and 10 minutes after injection of propofol with succinylcholine or thiopentone with succinylcholine respectively. RESULTS: There was significant increase in the serum potassium concentrations at 1, 5 and 10 minutes after injection of propofol-succinylcholine and thiopentone-succinylcholine compared with those before injection in two groups. No significant difference in serum potassium concentrations was observed between the two groups. CONCLUSIONS: The changes in serum potassium due to injection of propofol and succinycholine were very similar to those found during injection of anesthesia with thiopentone and succinylcholine. It was reassuring that such small changes of serum potassium occur when propofol-succinycholine and thiopentone-succinylcholine were used and that these changes within normal ranges were unlikely to be of clinical significance.