The pH of the gastric mucosal surface and gastric content was measured in 18 foals (mean age: 20 days) and 27 horses (mean age: 2.9 years) with a pH electrode passed through an endoscope biopsy channel. A reference electrode was attached to a shaved area on the neck. pH Measurements of the gastric mucosal surface at the dorsal squamous fundus (SF), squamous mucosa adjacent to the margo plicatus (MP), glandular fundus (GF) and the fluid or feed contents of the stomach were recorded in duplicate for each animal. In adult horses, the SF pH was greatest (5.46±1.82), with a decreasing pH ventrally toward the MP (4.12±1.62), to the glandular fundic mucosa (3.09±1.90), and fluid contents (2.72±1.86). The differences in pH at each site within the stomach were significant (P<0.05). In foals, mean pH measurement of the SF was 4.88±1.30, the MP was 4.92±1.29, the GF was 2.10±1.45, milk and fluid contents was 1.85±0.53 (six foals), and feed contents was 3.39±1.77 (12 foals). The pH of the SF and MP were significantly greater (P<0.05) than that of feed contents, which was significantly greater (P<0.05) than that of the glandular mucosal surface and fluid contents. The results indicate a dorsal to ventral pH gradient of the gastric mucosal surface in adult horses, and that gastric acid secretion is competent in young foals.