Since it was first discovered in the early 1980s, the role of atrial natriuretic peptide (ANP) in the control of fluid and electrolyte balance and blood pressure has been extensively studied in both health and disease. We report here a study of ANP and its relationship to corresponding changes in right atrial pressure (RAP) in patients with chronic renal failure (CRF) on haemodialysis compared to healthy controls. Although there was a positive correlation between RAP and ANP in both groups, the changes in ANP following changes in RAP between the two groups were not statistically significant. A unique observation was the response of RAP to changes in posture, with RAP falling significantly as expected in healthy controls in contrast to the exceptional absence of a significant fall in patients with CRF. Healthy controls demonstrated appropriate postural changes in plasma renin activity (PRA) despite marked suppression of PRA levels due to salt loading, in complete contrast to patients with CRF who, despite chronic fluid overload and elevated levels of ANP, continued to have grossly elevated PRA levels that failed to change significantly in response to changes in posture.