Atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) are cardiac hormones that regulate blood pressure and volume, and exert their biological actions via the natriuretic peptide receptor-A gene (Npr1). Mice lackingNpr1 (Npr ^−/−) have marked cardiac hypertrophy and fibrosis disproportionate to their increased blood pressure. This study examined the relationships between ANP and BNP gene expression, immunoreactivity and fibrosis in cardiac tissue, circulating ANP levels, and ANP and BNP mRNA during embryogenesis inNpr1 ^−/− mice. Disruption of theNpr1 signaling pathway resulted in augmented ANP and BNP gene and ANP protein expression in the cardiac ventricles, most pronounced for ANP mRNA in females [414 ± 57 inNpr1 ^−/− ng/mg and 124 ± 25 ng/mg in wild-type (WT) by Taqman assay, P < 0.001]. This increased expression was highly correlated to the degree of cardiac hypertrophy and was localized to the left ventricle (LV) inner free wall and to areas of ventricular fibrosis. In contrast, plasma ANP was significantly greater than WT in male but not femaleNpr1 ^−/− mice. Increased ANP and BNP gene expression was observed in Npr1 ^−/− embryos from 16 days of gestation. Our study suggests that cardiac ventricular expression of ANP and BNP is more closely associated with local hypertrophy and fibrosis than either systemic blood pressure or circulating ANP levels.