In order to evaluate the physiological relevance of the blood platelet "transient aggregation resistance" (TAR) test we studied the effect on this test of two different amounts of fish oil, corresponding to .75 g g (2.5 mmol) and 1.5 g (5 mmol) of eicosapentaenoic acid respectively, added in a cross-over design to the normal diet of 16 healthy male volunteers. It appeared that the "baseline aggregation resistance" (BAR), equivalent to the classic platelet aggregation ADP-threshold, was not influenced by Maxepa while, in contrast, a significant prolongation of TAR occurred. Apparently platelet aggregation analysed early after blood withdrawal, measures aspects of physiological relevance which, due to their short half life, are missed in the original method.