Atrial flutter/fibrillation (AF) are more readily inducible by atrial premature complexes (APCs) from the high right atrium (HRA) than from the coronary sinus (CS). Patients (pts) who have clinical AF have an exaggerated intraatrial conduction delay(IACD) and prolonged relative refractory period (RRP) in response to APCs delivered from the HRA. We therefore compared the response of APCs delivered from the HRA, posterior Triangle of Koch (SP), and CS to evaluate whether the propensity of HRA stimuli to induce AF is related to site-specific anisotropic conduction abnormalities. Programmed atrial stimulation at twice threshold was caried out in five pts from the HRA, SP, and CS. Recordings were made at the HRA, Sp, along the Tendon of Todaro (HBE catheter), and the CS. Pts were 3 women and 2 men, age 50 to 79, none of whom had clinical AF or organic heart disease. None had IACO during sinus rhythm. The RRP was defined as the range of intervals over which A1A2 was >S1S2 at the stimulus site, and A1A2 at distance sites was >A1A2 at stimulus sites. ΔCT denotes the maximum increase in conduction time at each site. Recording site ΔCT RRP Stimulation site HRA CS * SP HRA CS * CS * HRA 0 23 25 53 30 52 SP 65 24 0 82 30 32 HBEd 53 28 46 82 30 58 HBE2 52 28 33 78 28 58 HBE3 53 28 38 78 28 52 CSp 62 19 34 76 28 64 CS4 67 16 34 88 26 50 CS3 71 2 39 86 24 54 CS2 65 0 46 86 10 60 * p < 0.03, Data are means (in msec) AF was induced in 2 pts from the HRA but not from other sites. Our results demonstrate the greatest degree of IACD and RRP was seen with HRA stimulation and the least with CS stimulation. SP stimulation resulted in intermediate results. The marked site-specific difference in IACD supports anisotropic impulse conduction in the atria, and may explain why AF is more often initiated from the HRA tha n the CS.