Summary: Weaning can be a stressful period in foal's life resulting in increased prevalence of gastric ulcers which may lead to impaired rearing performance and economical losses. Beside medical treatment and optimizing housing and feeding conditions, dietary supplements have been proposed to reduce gastric mucosal lesions. Sixty-one warmblood weanlings from one stud farm, aged between 6 and 8 months were included in the study. During the experimental period, all foals were fed a total mixed ration (TMR) and hay ad libitum. After weaning, all foals were randomly and blindly allocated to one of three supplementation groups: group 1, control foals with no supplements; group 2, supplemented with MgO; and group 3, supplemented with a fermented preparation of herbs. Gastroscopy was performed prior to weaning and again after 16 days of feeding. Before weaning, the prevalence of gastric mucosal lesions was 82 % mainly found in the Curvatura minor of the Pars nonglandularis. After weaning, the prevalence was 96.7 %. The Antrum pyloricum and the Curvatura minor of the Pars nonglandularis showed a significant increase in gastric mucosa lesions after weaning in all groups. In both treatment groups, weaning resulted in a significantly higher lesion score in the Pars nonglandularis of the Curvatura major and on the pylorus and did not reduce the gastric lesion scores.