Previous in vitro studies showed that epinephrine stimulation can induce atrial natriuretic factor (ANF) release only form the right atrium but not from the left. In addition, sinus node has been shown to play an important role in the release of ANF. In vitro studies were done in isolated left and right rat atria to determine if pacing can induce the left atria to release ANF during epinephrine stimulation. ANF concentrations in the perfusate were measured by a radioimmunoassay method. Epinephrine increased ANF release in the right atria (from 6.3 ± 0.8 to 10.8 ± 0.9 pg/min/mg), but not in the unpaced left atria (4.2 ± 0.4 and 4.2 ± 0.3 pg/min/mg). However, when the atria were paced, ANF release rose in both the left (from 6.2 ± 0.5 to 11.5 ± 1.4 pg/min/mg) and right (from 8.4 ± 1.15 to 16.6 ± 1.8 pg/min/mg) atria with epinephrine addition. These results suggest that atrial contraction and tension play an important role in epinephrine-stimulated ANF release.