The purpose of the present study was to determine if ANF and the NH2-terminus of the ANF prohormone are secreted simultaneously in response to atrial distension in isolated perfused rat atria. The experiments were conducted in paced left atria perfused with a modified Krebs buffer. Atrial pressure was increased from a baseline level of 2 mmHg to 8-9 mmHg for 60 minutes (distension) and then returned to 2 mmHg for 60 minutes in one group (n = 9) of isolated atria. In a second group of atria (n = 6), atrial pressure was maintained at approximately 2 mmHg throughout the experimental period. ANF secretion averaged 100 pg/min during the three 10-minute periods immediately preceding the increased atrial pressure and increased to 600-800 pg/min (P less than 0.001) when atrial pressure was raised. Secretion of the proANF 31-67 peptide increased from a value of approximately 100 pg/min immediately prior to distension to a peak of over 200 pg/min during distension (P less than 0.01). Secretion of proANF 1-98 increased from an average of 1.25 ng/min during the three periods immediately prior to distension to a peak of 2.5 ng/min during distension (P less than 0.01). These data indicate that ANF and the NH2-terminus of the ANF prohormone appear to be simultaneously secreted by isolated paced atria.