Serum gastrin (A) and microdialysate histamine (B) concentrations in response to treatment with omeprazole (OPZ), YF476 (YF), OPZ+YF or vehicle (control group), starting at zero time. OPZ was given daily by the oral route, YF was deposited subcutaneously once. OPZ, YF and OPZ+YF induced a slow, progressive increase in the serum gastrin concentration for about a week. Subsequently, the elevated serum gastrin concentration remained at a plateau for the duration of the experiment. The microdialysate histamine concentration reached a peak after 7 days of OPZ treatment and declined thereafter. The concentration at day 7 was compared to the concentrations at days 14 and 28, **P<0.01, (Dunnett's test). The microdialysate histamine concentration of the control rats was reduced by treatment with YF476 (*P<0.05, and the effect of OPZ was prevented by YF. Means±s.e.mean (8 – 20).