Studies have been performed on man after truncal vagotomy to ascertain the effect of 1.2 mg of atropine sulfate on basal and postprandial immunoreactive gastrin. Atropine had no effect on basal gastrin, but it caused a significant increase in both the peak postprandial gastrin (135 to 240 pg per ml without the 139 to 308 pg per ml with atropine) and the integrated gastrin response (10.2 and 15.5 ng-min per ml over 2 hr respectively). This indicates that vagal integrity is not essential for the enhancement of the gastrin response by atropine and implies a direct effect on the antral gastrin cell.