Upregulation of protein kinase C (PKC), an important enzyme in platelet activation, could be one step toward platelet hyperactivity. PKC activation can be modulated by dietary components in vitro, but few data are available concerning the in vivo effects. In this strictly controlled human dietary intervention, the influence of dietary unsaturated fatty acids and vegetable compounds on platelet activation was investigated. A high linoleic acid diet (10% of energy) with small amounts of vegetables (no berries or apples) was consumed by 9 women and 4 men (24.1 +/- 3.9 y), and was compared with a high oleic acid diet (12% of energy) with considerable amounts of vegetables, berries and apples consumed by 8 women and 4 men (24.2 +/- 5.5 y). Subjects were healthy Finnish volunteers. Compliance with the experimental protocol was good, as indicated by changes in plasma fatty acids and concentrations of vitamin C, beta-carotene and alpha-tocopherol. No differences between groups were seen in indices of platelet activation, including platelet aggregation, total PKC activity and distribution of PKC isoenzymes alpha, beta(II) and delta. The results indicate that in apparently healthy and fairly young subjects with adequate vitamin intakes, diets differing markedly in their amounts of linoleic and oleic acids, and vegetables, berries and apples do not differ in platelet activation.