Ulceration of the gastric squamous epithelial mucosa was induced in 10 horses using a feeding protocol previously shown to expose the gastric mucosa to repeated periods of high acidity. The feeding protocol consisted of alternating feed deprivation with free access to hay. Over a period of seven days, each horse was provided hay for 84 hr and deprived of hay for 84 hr. Hay was never withheld for longer than 24 hr at a time. Gastroscopy was performed on each horse at the beginning of the protocol after 12 hr of feed deprivation, and after a total of 36 hr, 60 hr, and 84 hr of feed deprivation. All horses had normal esophageal and gastric mucosa at the beginning of the protocol. Alternating periods of feeding and feed deprivation resulted in progressive ulceration of the gastric squamous epithelial mucosa in all but one of the horses. Lesions progressed from erosions to ulceration within 36-72 hr of feed deprivation, and by 84 hr of feed deprivation ulcers had developed thickened, raised margins. Esophageal mucosa appeared normal in all horses throughout the study. The protocol of alternating feeding with feed deprivation in horses consistently produced gastric squamous epithelial ulceration and provided a useful model for characterizing temporal transitions in peptic-injured alimentary squamous epithelia.