BACKGROUND The PNA (protein equivalent of nitrogen appearance) is used to calculate protein intake from urea kinetics. One of the essential assumptions in the calculation of PNA is that urea accumulation in haemodialysis (HD) patients is equivalent to amino acid oxidation. However, urea is hydrolysed in the intestine and the resulting ammonia could be used metabolically. The magnitude and dependence on protein intake of this process are unknown in HD patients. METHODS Seven HD patients were studied twice, 1 week apart, on a similar protocol. After an overnight fast, patients fasted in the morning and received meals in the afternoon. On one day, amino acid oxidation was measured by infusion of L-[1-(13)C]valine. Urea production, measured from the dilution of [(13)C]urea, and urea accumulation, calculated from the increase in plasma urea concentration multiplied by the urea dilution volume, were measured during the other day. PNA was calculated using standard equations. RESULTS Amino acid oxidation and urea production were not significantly different during fasting. Urea accumulation during fasting was significantly lower than both amino acid oxidation and urea production. Urea accumulation during feeding remained significantly lower than amino acid oxidation. PNA was equal to the average of the urea accumulation values during fasting and feeding. CONCLUSIONS We conclude that during fasting, urea accumulation is not associated with amino acid oxidation or urea production. During meal intake, amino acid oxidation, urea production and urea accumulation show acutely an almost identical increase. PNA represents the average of fasting and fed urea accumulation and is lower than average amino acid oxidation or urea production.