A feeding trial involving four Thoroughbred race horses was undertaken to establish whether inclusion of grape seed extract (GSE) in the diet of horses undergoing mild exercise had any effects on their general health, intake and digestion. Supplementation with GSE had no effect on either feed or water intake of the horses and the supplement was readily palatable to the horses at all levels of inclusion. Feeding GSE caused no adverse effects in terms of aniaml health (temperature, pulse and respiration rates), and there were some positive effects related to a presumed alteration in fermentation in the hindgut. Feeding GSE increased faecal pH, changing from acid faeces (pH 6.6) when no GSE eas fed to neutral faeces (pH 7.0) when 150 mg GSE/kg body weight (BW) was fed. In addition, blood glucose concentrations were significantly (P<0.05) decreased when GSE was fed at 100 and 150 mg/kg BW (5.50 +/- 0.26 and 5.32 +/- 0.72 mmol/l, respectively) compared with the control diet (5.77 +/- 0.31 mmol/l). The actual mechanisms causing these alterations are yet to be elucidated, but could have important implications for the prevention of acidosis.