Organosulfur compounds in onion extracts are formed following the lysis of the S-alk(en)yl-L-cysteine sulfoxides by alliinase. These compounds inhibit the aggregation of human blood platelets and offer the potential for positive cardiovascular health benefits. An experiment was designed to examine temporal and temperature effects on onion-induced antiplatelet activity. Platelet aggregation is induced by various agonists, including ADP, collagen, and thrombin. Unexpectedly, all freshly-juiced onion extracts (ca. 5 minutes post-juicing) appeared to exhibit both an agonist-free aggregation peak (AFP) and a platelet inhibitory peak (PIP) characteristic of inhibition of platelet aggregation. The AFP was minimal by 30 minutes and dissipated in all treatments by 120 minutes, while the PIP increased as onion extracts aged and did not change after 30 minutes at 25 degrees C. This finding confirms the observation that the in vitro platelet inhibitory activity of onion organosulfur compounds is time dependent. Freshly-prepared onion extracts were incubated with the ADP scavenger enzyme apyrase (E.C. 188.8.131.52). AFPs were abolished in apyrase-treated extracts, suggesting that this response may have been due to free ADP in onion extracts, although an amount of ADP required to generate such a response would be unexpected in onion extracts. In addition, platelet aggregates were not observed in the AFP, suggesting this response may be associated with changes in light transmission through platelet rich plasma that are not associated with platelet aggregation. Artifacts of analysis are, therefore, possible when assessing onion-induced antiplatelet activity with freshly-juiced extracts. Temporal formation of platelet-inhibiting organosulfur compounds should be taken into account during both in vitro and in vivo assessment of onion-induced antiplatelet activity.