The effect of an endurance physical training programme on the plasma and atrial natriuretic peptides (ANP) and on renal glomerular ANP receptors was evaluated in male normotensive Wistar rats. Maximal O2 uptake was significantly greater in the endurance trained (117.1 ml O2.kg-1.min-1, SEM 6.18 versus the control rats 84.2 ml O2.kg-1.min-1, SEM 4.88, P less than 0.01. In addition, various muscle oxidative enzymes were also significantly higher in endurance trained animals. An increase in resting plasma [ANP] was observed after 11 weeks of physical training (40.02 pg.ml-1, SEM 7.07 vs 22.8 pg.ml-1, SEM 3.83, P less than 0.05). Glomerular ANP receptor density was lower in trained rats (272 fmol.mg-1 protein, SEM 3.1 vs 380 fmol.mg-1 protein, SEM 6.1, P less than 0.05), whereas atrial tissue [ANP] was not significantly different between controls and trained animals. However, in trained rats, circulating [ANP] was closely correlated with left atrial [ANP] (r = -0.92, P less than 0.05). Resting systolic blood pressure had not changed at the end of this physical training programme. It is considered that under physiological conditions ANP may be involved in long-term extracellular fluid volume homeostasis through the regulation of renal glomerular ANP receptors, and that the left atrium might play a significant role in this long term fluid volume control.