Prolonged achlorhydria leads to hypergastrinemia which must be matched by increased gastrin production. The extent to which the balance between synthesis and storage or secretion is shifted in achlorhydria remains uncertain. In the present study, rats were treated for 14 days with the hydrogen-potassium-stimulated ATPase inhibitor omeprazole, and the effects on plasma and tissue gastrin concentrations and on the abundance of gastrin messenger RNA were examined. To calculate the fractional release rates of gastrin, the metabolic clearance rate of synthetic unsulfated rat heptadeca peptide gastrin in anesthetized rats was also measured. Treatment with omeprazole for 14 days led to a profound hypergastrinemia, a twofold increase in antral gastrin stores, and a tenfold increase in messenger RNA. Calculations based on the metabolic clearance rate for rat heptadecapeptide gastrin suggested that in control rats, about 0.08% of stored gastrin was released per minute compared with about 0.4% in omeprazole-treated rats. No evidence was observed to suggest that changes in the efficiency of conversion of Gly-extended gastrins to amidated peptides were of any significance in accounting for the increased production of amidated gastrin. The increased gastrin synthesis in achlorhydria is therefore attributable to increased messenger RNA levels; most of the increase in gastrin production is directly secreted as changes in the stores of gastrin appear to be of lesser importance.