Spontaneous and experimental rises of intracardiac pressure and/or volume increase the level of atrial natriuretic (ANP) mRNA in rat atrial tissue. There is expanding evidence that ANP synthesis is increased in the ventricle under such conditions. However, little is known with regard to the myocardial ANP synthesis response to physical training. In this study, plasma and atrial immunoreactive ANP concentrations were measured in Sprague-Dawley rats trained on a treadmill and compared to sedentary controls. Atrial natriuretic peptide mRNA was detected in the heart cavities of each group by dot-blot hybridization analysis. Physical training reduced the mean immunoreactive ANP plasma levels from 405 +/- 99 to 303 +/- 45 ng/l (p < 0.05). Immunoreactive ANP in the left atrium was depleted after endurance training, while immunoreactive ANP concentration in the right atrium was unaffected. Physical training resulted in a 70% (p < 0.01) rise in ANP mRNA of the right atrium, while no changes in the other compartments were found. These data indicate that during physical training: ANP mRNA does not increase in ventricles; despite depletion of immunoreactive ANP in the left atrium, no corresponding changes of ANP mRNA are detected; and ANP mRNA increases in the right atrium while its immunoreactive ANP does not. These findings suggest that during chronic exercise the ratio between immunoreactive ANP and ANP gene expression in the atria may be altered.