1. Homozygous deletion of the pro-atrial natriuretic peptide (Nppa) gene (ANP-/-) has been associated with both cardiac hypertrophy and salt-sensitive hypertension in mice, suggesting that cardiac hypertrophy in ANP-/- mice may be related, at least in part, to increased afterload. 2. To test the hypothesis that cardiac hypertrophy in ANP-/- mice is independent of blood pressure, male ANP-/- and wild-type ANP+/+ mice were fed a low (0.05%) or basal (0.55%) NaCl diet. Five weeks later, mean arterial pressure (MAP) was measured in conscious mice; the whole heart, atria, left and right ventricles (LV and RV, respectively), brain, lung, kidney, liver and spleen were weighed and fixed for histological analysis. Separate groups of mice were subjected to echocardiographic examination under tribromoethanol anaesthesia. 3. Mean arterial pressure and atrial, LV and RV mass were greater in ANP-/- mice than in ANP+/+ mice fed the basal salt diet. Salt depletion equalized MAP in the two genotypes, but did not alter the relative cardiac hypertrophy in ANP-/- mice. The ANP-/- mice had significant LV cardiomyocyte hypertrophy when fed either basal or low-salt diets. 4. Left ventricle chamber dimensions did not differ between genotypes, but were significantly reduced in mice fed the low-salt diet; LV posterior wall and septal thickness were greater in ANP-/- than ANP+/+ mice and were not altered by diet, indicating a concentric pattern of LV hypertrophy in ANP-/- mice. Left ventricle function (cardiac output, stroke volume, ejection fraction, circumferential wall stress and velocity of circumferential wall shortening) did not differ between strains on either diet; circumferential wall stress was reduced in the low-salt groups; other functional parameters were not altered by diet. 5. These findings indicate that ANP deletion results in cardiomyocyte hypertrophy and biventricular hypertrophy independent of blood pressure, supporting the concept that ANP has direct antihypertrophic effects in the heart.