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Tachycardia is known to increase the plasma level of atrial natriuretic factor (ANF). The aim of the study was to determine whether such release of ANF can occur independently of increased atrial wall stretch. Seven anesthetized dogs were instrumented with pressure catheters and sonomicrometer crystals to measure left and right auricular and atrial free wall dimensions. During atrial pacing, the measured atrial dimensions were kept constant by adjusting vascular constrictors placed around the great vessel trunks. When atrial rate was increased progressively by pacing at 150, 200, and 250 min-1, plasma ANF increased from 40.8 +/- 5.4 to 43.0 +/- 6.0 (P = NS) and 103.1 +/- 17.6 (SE) pmol/l (P less than 0.05), respectively. Repeating the pacing sequence at two different levels of elevated left atrial pressure caused at each level a similar increase in plasma ANF at 250 min-1. The relationship between plasma ANF and atrial dimensions was shifted upward by pacing, such that ANF was higher for any given atrial dimension. In conclusion, pacing tachycardia elevates plasma ANF in the presence of constant atrial dimensions. These results support the hypothesis that chronotropic stimulation can cause release of ANF by a mechanism that is independent of atrial stretch.