BACKGROUND: The purpose of this study was to assess the effect of rocuronium pretreatment on the succinylcholine induced biochemical changes and fasciculations, myalgia and to compare it with vecuronium pretreatment. METHODS: We have studied 60 patients undergoing minor elective surgery, in a prospective double blinded method. Three groups of 20 patients each were pretreated with saline 0.01 ml/kg (group C), rocuronium 0.05 mg/kg (group R), or vecuronium 0.007 mg/kg (group V). Three minutes after the pretreatment, 1.5 mg/kg succinylcholine was injected. Serum potassium five minutes after succinylcholine and creatine kinase 24 hr after operation were measured. Fasciculations and postoperative myalgia at 24 and 48 h were evaluated. RESULTS: The increase in creatine kinase and incidence of fasciculations were lowest in the rocuronium group (33 IU/L; 15%) and followed by vecuronium group (58 IU/L; 50%) and highest in the control group (101 IU/L; 100%). The increase in serum potassium concentration (0.3 mEq/L) was not attenuated by any regimen. The incidence of postoperative myalgia on day 1 and day 2 was lower in the rocuronium (40%, 30%) and the vecuronium (50%; 35%) group than control group (85%; 75%). CONCLUSIONS: Rocuronium pretreatment is more effective in reducing creatine kinase rise and fasciculations after succinylcholine than vecuronium. However rocuronium and vecuronium are also effective in preventing postoperative myalgia.