Seventeen Thoroughbred horses with moderate to severe gastric ulceration were purchased from a race track within 10 days of racing and were treated once daily with either omeprazole (9 horses) or vehicle (8 horses) and evaluated gastroscopically for ulcer healing. Horses were administered omeprazole (1.5 mg/kg bwt/day) or vehicle by nasogastric tube once daily. Gastroscopic examination was performed on Days 0, 4, 7, 11, 14, 17, 21, 24 and 28, until lesions healed completely. Selected images of gastric lesions were captured by computer at each endoscopic examination, with a measuring caliper included in captured images. The area and perimeter of lesions were measured by computer and healing rates of specific lesions were determined by calculating the rate of linear advance of the margins toward the centre of the lesion. Additionally, the number of days to complete healing of the entire gastric squamous mucosa was compared between treatment groups. Gastric lesions healed at a significantly faster rate in horses receiving omeprazole than in vehicle-treated horses (P < 0.001). Complete healing of the entire stomach occurred in 10-21 days in omeprazole-treated horses, and 14-28 days in 3 of 8 vehicle-treated horses, with the remaining vehicle-treated horses having unhealed lesions on Day 28. In addition, 5 vehicle-treated horses developed new lesions in the squamous epithelial mucosa during the trial; no new lesions were observed in the omeprazole-treated group.