The purpose of this study is to elucidate factors determining the release of atrial natriuretic peptide (ANP) in normal volunteers at rest. Ten normal volunteers were included in this study. Ages ranged between twenty-seven and thirty-three years (mean twenty-seven). The authors measured plasma levels of ANP, and four cardiac chamber volumes, and their functions by cine magnetic resonance imaging using the rephased gradient-echo method. Plasma levels of ANP in 10 normal subjects were 15.0-37.1 pg/mL (mean 23.6 pg/mL). No significant relationship was seen between ANP level and heart rate or blood pressure. The level of ANP was positively correlated with left atrial (LA) emptying fraction and negatively correlated with LA minimal volume (r = 0.85, P < 0.01; r = 0.64, P < 0.05, respectively). No significant relationship was observed between plasma ANP level and left ventricular, right ventricular, or right atrial parameters. These results suggest that LA systolic function is one of the major determinants for ANP release in normal subjects at rest.