Platelets play a substantial role in cardiovascular disease, and for many years there has been a search for dietary componentsthat are able to inhibit platelet function and therefore decrease the risk of cardiovasculardisease. Platelets can be inhibited by alcohol, dietary fats and some antioxidants, including agroup of compounds, the polyphenols, found in fruits and vegetables. A number of these compounds have been shown to inhibit platelet function both in vitro and in vivo. In the present study the effects of the hydroxycinnamates and the flavonoid quercetin on platelet activation and cell signalling in vitro were investigated. The hydroxycinnamates inhibited platelet function, although not at levels that can be achieved in human plasma by dietary intervention. However, quercetin inhibited platelet aggregation at levels lower than those previously reported. Quercetin was also found to inhibit intracellular Ca mobilisation and whole-cell tyrosine protein phosphorylation in platelets, which are both processes essential for platelet activation. The effect of polyphenols on platelet aggregation in vivo was also investigated. Twenty subjects followed a low-polyphenol diet for 3 d before and also during supplementation. All subjects were supplemented with a polyphenol-rich meal every lunchtime for 5d. Platelet aggregation and plasma flavonols were measured at baseline and after 5d of dietary supplementation. Total plasma flavonoids increased significantly after the dietary intervention period (P = 0.001). However, no significant changes in ex vivo platelet aggregation were observed. Further investigation of the effects of individual polyphenolic compoundson platelet function, both in vitro and in vivo, is required in order to elucidate their role in the relationship between diet and the risk of cardiovascular disease.