The purpose of the study was to assess at rest and during exercise total sympathetic activity, as expressed by plasma cyclic AMP (cAMP) blood levels and sinus node activity (SNA), as well as atrial natriuretic factor (ANF) blood levels in VVI and DDD pacing with long and short atrioventricular delays in DDD paced patients suffering from complete heart block. Clinical parameters, such as exercise time, and arterial blood pressure (ABP) were also taken into consideration. Thirteen patients (six males, mean age 65 +/- 2 years), were examined randomly in VVI and DDD pacing with 100 and 150 ms atrioventricular delays (AVD). Plasma cAMP and ANF were measured at rest, at peak exercise and 15 and 30 min after the test. The cAMP at rest remained unchanged whatever the pacing mode or the AVD, but 30 min after exercise, the cAMP levels were statistically lower in DDD pacing with short AVD (100 ms) than in DDD with long AVD (150 ms) or VVI pacing (cAMP DDD/100 ms: 16 +/- 0.8 pmol.ml-1, cAMP DDD/150 ms: 20 +/- 2 pmol.ml-1, P < 0.01, cAMP VVI: 29 pmol.ml-1, P < 0.001). ANF plasma levels at rest were significantly higher in VVI pacing than in DDD modes, with either long or short AVD (ANF DDD/100 ms: 93 +/- 10 pg.ml-1, ANF DDD/150 ms: 100 +/- 13 pg.ml-1, ANF VVI: 150 +/- 16 pg.ml-1, P < 0.001, P < 0.03 respectively compared to VVI).(ABSTRACT TRUNCATED AT 250 WORDS)