The relationship of fasting serum gastrin (FSG) levels to the histologic state of antral and body mucosa and to the stimulated acid output (PAO) was examined in 860 subjects. The FSG levels correlated with PAO and atrophy of the body mucosa: the FSG increased linearly with an increase in the grade of body atrophy and increased exponentially when the PAO decreased from 'normal' (greater than 10 meq/h) to zero. In subjects with achlorhydria or marked hypochlorhydria (PAO less than 1.1 meq/h) accompanying moderate or severe atrophy in the gastric body mucosa, FSG decreased linearly with increasing grade of atrophy in the antral mucosa. No such relationship between antral atrophy and FSG was found in subjects who had a PAO above 1.1 meq/h or who had non-atrophic gastric body mucosa. We conclude that the state of the antral mucosa influences the FSG level, but only when the function of antral G cells is maximal--that is, in achlorhydric or nearly achlorhydric conditions in which the inhibitory effect of intragastric acidity on the G cells' secretion of gastrin into the circulation is minimal.