Sixteen neonatal foals stressed by disease underwent endoscopic examination of their stomachs and blood was assayed for triiodothyronine (T3), reverse T3 (rT3), thyroxine (T4) and cortisol, to determine the effects of severe physiological stress and the occurrence of gastric ulcers. compared with eight age-matched controls, six foals had abnormal cortisol, seven had abnormal T3 and 12 had abnormal T4. Eleven of 13 foals had rT3 outside the 95 per cent confidence interval for clinically normal foals of comparable ages. Gastric lesions were seen more frequently in stressed foals, and gastric glandular mucosal lesions were noted in 40 per cent of the stressed foals. Previous studies report low (3 per cent) occurrence of gastric mucosal lesions. The frequency of squamous mucosal lesions was not different from that reported previously, indicating that stress has little effect on the development of lesions at this site.