The effect of prostaglandin E1 (PGE1) on basal and food-stimulated concentrations of serum gastrin was studied in dogs with denervated fundic (Heidenhain) pouches. PGE1 did not change basal levels of serum gastrin. When PGE1 was given with food, the Heidenhain pouch acid output was inhibited significantly. The serum gastrin response to food, however, increased after PGE1. The results suggest a direct inhibitory effect of PGEX on the parietal cells. Elevation of gastrin may be due to diminished inhibition of the antrum by acid, decreased antral motility, or alteration of catabolic pathways for gastrin (Endocrinology93: 1148, 1973)