We studied the acute effect of transthoracic truncal vagotomy or sham vagotomy (control) on fasting serum gastrin concentrations in 22 gastric fistula dogs. A significant (p less than 0.05) decrease in serum gastrin concentration was detectable within 2.5 minutes after truncal vagotomy, and by 120 minutes serum gastrin has decreased to 15 +/- 1 pg/mL in the vagotomy group compared to 28 +/- 3 pg/mL in the control group (p less than 0.001). However by 24 hours after vagotomy, when maximal acid output was reduced by approximately 50%, fasting serum gastrin had increased nearly twofold above control levels in the vagotomy group (p = 0.06) and this increase persisted at day 7 (p less than 0.05). Thus truncal vagotomy had a biphasic effect on serum gastrin concentrations in dogs (acute inhibition followed by stimulation). While the mechanism for the acute fall in gastrin is probably an acute denervation of postganglionic neurons that innervate gastrin cells, the mechanism for the subsequent rise in serum gastrin remains uncertain.