Although sinus node recovery time (SNRT) assessment by the overdrive suppression test (ODST) is important in detecting sick sinus syndrome (SSS), its sensitivity is still inadequate. We have evaluated the effect of intravenous injection (i.v.) of disopyramide phosphate (DP) in ODST. The subjects were 30 SSS patients (64.9 +/- 10.0 <SD> years old). If SNRT was <2,000 ms or the corrected SNRT (CSNRT) was < 1,000 ms, ODST was repeated after DP i.v. (2 mg. kg(-1), < or = 100 mg in total). Eleven normal subjects (59.3 +/- 9.0 years old) were also studied. Although SNRT was <2,000 ms or the CSNRT was < 1,000 ms in 13 of the 30 SSS patients (43%), SNRT was prolonged from 1,510 +/- 300ms to 3,400 +/- 1,160 ms (P<0.01), and CSNRT from 510 +/- 190 to 2,470 +/- 1,470 ms (P<0.01) after DP i.v. in these patients. Thus, SNRT was > or = 2,000 ms and the CSNRT was > or = 1,000 ms in 27 of 30 SSS patients (90%) after DP i.v. Using a combination of overdrive suppression and intravenous injection of disopyramide phosphate, the corrected sinus node recovery time was diagnostic (>525 ms) in 29 of the 30 patients (97%). In contrast, SNRT and CSNRT were shortened in the normal subjects during ODST after DP i.v. (P<0.01). The plasma concentration of DP estimated in nine patients was 4.1 +/- 1.0 microg.ml(-1). No serious side effect occurred. ODST employing DP i.v. is safe and seems to be highly effective in diagnosing SSS.