Extracts from seabuckthorn (Hippophea rhamnoides) leaves, fruit and seed oil were screened for anti-platelet properties in-vitro using human platelets stimulated with 0.2 mM ADP. Half maximal inhibitory concentration was found to be 55 microg/ml platelet rich plasma(PRP) for leaf extract, 47.7 microg/ml PRP for fruit extract and 0.62 microl/ml PRP for seed oil. In-vitro incubation of platelets with increasing concentrations of seed oil was found to inhibit oxidative stress in resting as well as agonist stimulated platelets as evident by decreased formation of peroxide and superoxide radicals. Leaf extract and seed oil were further evaluated for antiplatelet and antioxidant action in-vivo in cholesterol-induced experimental atherosclerosis in rabbits. Rabbits fed on diet supplemented with cholesterol (0.5% or 1%) for 60 days showed statistically significant increase in platelet aggregation stimulated by ADP and collagen as compared to platelet aggregation in control rabbits. Malondialdehyde levels in platelets of cholesterol fed rabbits were significantly higher than those of control rabbits. Both leaf extract and seed oil supplementation prevented cholesterol induced alterations in platelet aggregation and platelet malondialdehyde levels. The study shows that SBT leaf extract and seed oil inhibit oxidative stress and hyperactivity of platelets both in-vitro as well as in-vivo and plausibly attenuate atherogenesis.