Serial meanurements of the pacing threshold have been considered as essential for follow-up of the patients in whom the pacemaker had been implanted because pacing threshold is directly related to the success of long-term pacemaker therapy and reflects the alterations in electrobiologic factors influencing it. The development of the noninvasive technique of measuring pacing threshold such as Vario system made the noninvasive follow-up of it feasible and therefore has contributed to understanding of long-term threshold behavior. This study was performed to get the knowledge of acute and chronic pacing threshold behavior by measuring it serially in 46 patients after pacemaker implantation using a non invasive technique of Vario system. Patients subjected to the present study were 46(18 males, 28 females) comprising 21 sick sinus syndromes, 24 A-V blocks, and 1 combined disorder. All were received a multiprogrammable pacemaker of VVI mode (OPTIMA-MP, Telectronics). Pacing threshold was increased significantly from initial threshold(0.65+/-0.22) 2 days after implantation and reached to peak(1.65+/-0.75 volts) in the fourth week, thereafter it was maintained around twice the initial value. In the 30 patients followed more than 3 months, the maximum increase and difference in pacing thresholds were 0.86+/-0.62 volts and 0.93+/-0.56 volts respectively and the ratios of peak threshold and threshold at the end of follow-up to initial threshold were 2.56+/-1.23 and 2.30+/-1.30 respectively. Pacing threshold exceeded 2.0 volts in 7 patients(15.2%), but transiently in 3 of 5 patients in whom it happened within 6 weeks after implantation. Safety margins of long-term thresholds were acceptable(more than 3) in all patients at 5.0 volts and 19(63.3%) at 2.5 volts of programmed output.