We investigated the effect of raising arterial plasma epinephrine within the lower pathophysiological concentration range on various indicators of blood platelet function and hematocrit. Epinephrine was raised over 60 minutes by a stepwise increasing intravenous infusion in 40 healthy men aged 20 to 40 years. Platelet count increased progressively with increasing arterial epinephrine to a maximal change of 69±6 ×10⁹/L in EDTA-anticoagulated blood and a maximal change of 42±6 ×10⁹/L in acid-citrate-dextrose (ACD)–anticoagulated blood, and the weight of circulating platelets increased by 29% (P<.001). Platelet size increased significantly in EDTA and decreased in ACD, and the difference between EDTA and ACD was significant (P<.0001) for both count and size, suggesting that epinephrine not only recruits platelets into the circulation but also induces some microaggregation in vivo or adhesion ex vivo. Aggregation of platelets in vitro induced by epinephrine decreased (P<.003 for Δoptical density and P=.038 for maximal optical density) after epinephrine infusion compared with saline but did not change when stimulated with ADP or collagen. These findings suggest a selective downregulation of the epinephrine-activating mechanisms concomitant with a rise in the platelet content of epinephrine by 81% (P<.001) and no change in the platelet sodium-proton membrane exchange. The release of granular content (β-thromboglobulin and platelet factor 4) to the circulation in response to epinephrine was not significant. Thus, under acute conditions it seems that the platelets may protect themselves against inappropriate overstimulation by epinephrine. The importance of platelet epinephrine uptake is still unknown, but sodium-proton exchange does not seem to be involved in regulating the effects of circulating epinephrine on platelet function. Epinephrine has a pronounced effect on raising hematocrit (maximal change of 1.74±0.13 ×10⁻², P<.0001).