Abstract: This prospective, randomized, double blind, placebo controlled study was designed to examine the safety and impact of extended esmolol infusion on plasma choline esterase (PChE) activity and mivacurium neuromuscular blockade and cardiovascular profile. Forty adult ASA I/II adult patients undergoing elective endo-urological surgery were included. Patients were randomly assigned to receive at induction either esmolol or saline infusion. All patients received fentanyl-propofol mivacurium (0.2 mg kg-1) induction and isoflurane in oxygen maintenance of anaesthesia. Mivacurium infusion was adjusted to maintain the twitch response at 5-10% of the control. The PChE activity was more inhibited (by >2.5 times) in the esmolol group compared to the control group. The onset times were similar in the two groups. There was significant prolongation of mivacurium clinical duration (by 18.6%) and recovery index (6.67%) as well as, significant reduction of the total mivacurium consumption (by 24.1%) in the esmolol group compared to the control group. The heart rate and blood pressure in the esmolol group were significantly lower than those in the control group throughout the study period. This study demonstrate that esmolol infusion caused a moderate inhibition of the PChE activity resulting in moderate prolongation of the clinical duration and recovery index of mivacurium and reduction of its total dose consumption by infusion without the affecting the onset time. Caution is warranted if this combination is used in a patient with low PChE activity due to a disease, e.g., severe liver impairment or drugs, e.g., bambuterol.