The effect of selective right or left atrial appendectomy on atrial natriuretic factor (ANF) release was studied in the rat to compare the relative contribution of each atrium in a situation of increased afterload. Sham-operated or right (RAA) or left (LAA) atrial appendectomized conscious rats were infused (intravenously) for 60min with a pressor dose of angiotensin II (Ang II; 600 ng/kg per min). All groups presented a significant and similar elevation of mean arterial pressure (MAP) and left ventricular end-diastolic pressure (LVEDP). No change in central venous pressure (CVP) was noted in the LAA group, whereas a small, transient rise was observed in the two other groups. The Ang II-induced release of ANF C-terminal was blunted in the RAA group (P < 0.01 versus sham) and more so in the LAA group (P < 0.001 versus sham). Atrial natriuretic factor N-terminal release was blunted only in the LAA group (P < 0.05 versus sham). No correlation was found between CVP and ANF in any group. The relationship between LVEDP and ANF in the sham-operated controls became weaker in the LAA group and was lost in the RAA group. We conclude that both atria play a role in the ANF release induced by a vasoconstrictor agent, but changes in left heart haemodynamics may be the first to be involved in the response to an increased afterload.