It is important to have a reliable method of assessing the dietary nitrate exposure of populations for a proper understanding of the potential health effects of the endogenous metabolites of this ion to be gained from epidemiological studies. Recently we strongly advocated the use of the nitrate analysis of 24 h urine samples as being a superior method for such studies. Our previous observations and those of others relating to nitrate pharmacology in healthy human volunteers formed the basis of this judgement. The purpose of this study was to determine whether gastric achlorhydria or hypochlorhydria has any significant gross effects on the urinary recovery of dietary nitrate and to what extent the inclusion of such individuals would compromise the results of potential epidemiological studies. The results demonstrated a significantly greater loss of dietary nitrate as measured by urinary recovery in achlorhydrics than in normochlorhydrics, presumably as a consequence of bacterial metabolism in the colonized stomach. Thus the average urinary nitrate recovery of a 1.5 mmol challenge was 33% in individuals with reduced stomach acidity as compared to 56% in normal controls. This significant further loss of nitrate (average 23%) when intragastric conditions favour bacterial colonization clearly indicates that for valid assessments and comparisons of nitrate exposure between populations it would be wise to exclude individuals with low levels of stomach acid where this is likely to lead to significant gastric colonization by nitrate-reducing bacteria (i.e. pH less than 4-5).