AIM--To study the effect of known interference in the measurement of urea and ammonium concentrations in samples of gastric juice. METHODS--The effect of pH and ammonium concentration on the o-pthalaldehyde method, the diacetylmonoxime method, a Berthelot linked method and an enzymatic urease method for the measurement of urea in gastric juice was therefore conducted. An enzymatic method of the measurement of ammonium in gastric juice was also assessed. RESULTS--The o-pthalaldehyde and the enzymatic urease methods were unaffected by a low gastric juice pH, ammonium concentrations of 10 mmol/l, and had interassay coefficients of variation of 3.9-5.6% and 2.8-10.6%, respectively, over a urea concentration of 2.5 mmol/l-20 mmol/l. The Berthelot linked method resulted in low gastric juice urea concentrations. The enzymatic method of ammonium measurement also proved suitable when the effect of low gastric juice pH was controlled. CONCLUSION--Interference by low pH did not explain the differences in reports of gastric juice urea or ammonium concentrations.