The effect of age on atrial natriuretic factor (ANF) metabolism in the rat was studied. Plasma ANF levels rose with age. A positive correlation was found between age and plasma ANF levels. Molecular forms of ANF were also examined. α-ANF and γ-ANF were present in the circulation. The ratio of γ-ANF/α-ANF increased with age. The atria were isolated and perfused to determine the ANF secretion rate. The spontaneous release of ANF decreased with age. The released ANF mainly consisted of α-ANF with a small amount of γ-ANF. The ratio of γ-ANF/α-ANF in the effluent increased with age. ANF concentrations in the atria varied with age, but the molecular forms did not. γ-ANF was the main form found in the atria. These results suggest that ANF secretion as well as post-transcriptional processing is altered with age.