A heterologous radioimmunoassay was used to measure the concentration of immunoreactive atrial natriuretic peptide (iANP) in plasma from the femoral artery of eight chloralose anaesthetized dogs. Mitral obstruction which increased left atrial pressure by 11 cmH2O increased plasma iANP from 97 ± 10.3 (mean ± SE) to 135 ± 14.3 pg/mL. Pulmonary vein distension increased heart rate but did not increase plasma iANP. Bilateral cervical vagotomy and administration of atenolol (2 mg/kg) did not prevent the increase in iANP with mitral obstruction. Samples of blood from the coronary sinus had plasma iANP significantly higher than simultaneous samples from the femoral artery confirming the cardiac origin of the iANP. Release of iANP depends on direct stretch of the atrium rather than on a reflex involving left atrial receptors.