Expansion of extracellular fluid volume,--increasing right atrial pressure,--appears to be a major stimulus for the rise in plasma atrial natriuretic peptide (ANP). End-stage renal disease (ESRD) is associated with significant changes of several hormone systems. Recent studies have shown that plasma ANP concentration is significantly increased in patients with ESRD and that the concentration of the hormone declines following haemodialysis (HD). In patients with ESRD treated by HD "dry weight" for each individual patient is the body weight at which the patients has normal hydration status. The aim of the present study was to determine changes of plasma concentrations of ANP during haemodialysis. We also attempted to check whether the baseline levels of ANP were related to weight loss during dialysis. Eleven children (7 females, 4 males) participated in the study. The patients age related from 8 to 20 years. Plasma concentrations were markedly elevated (p < 0.01) before haemodialysis (251 ae 119) pg/ml) in comparison with control values (61 ae 23 pg/ml), but remained above normal range in all except 4 cases. During HD body weight was reduced in all but one patient. This patient was infused isotonic fluid during HD and did not show weight loss. The four patients who had normal plasma ANP levels at the end of treatment appeared to be in better overall condition than the other patients. In summary, plasma ANP measurements may be of value in establishing the volume status in patients with ESRD. Plasma ANP levels after HD may help to adjust the right dry body weight for each patient.