1. Ligation and division of the vessels supplying the stomach, except those from the spleen, result in greatly increased pepsin secretion in response to pentagastrin, pilocarpine or 2-deoxy-D-glucose. 2. This effect was still present 6 months after initial ligation. 3. Analysis of dose-response curves shows that for pepsin secretion the Vmax values were raised by this procedure. 4. Right-sided vessel ligation and division does not alter acid secretion in response to the stimuli used. 5. It is suspected that the procedure of right-sided vessel ligation removes a non-competitive inhibitor mechanism to gastric pepsin secretion.