Catecholamines and other catecholic compounds have opposite effects on the cyclooxygenase and 5-lipoxygenase pathways of arachidonic acid metabolism in human polymorphonuclear leukocytes and whole blood in vitro. The hypothesis that high levels of adrenaline, found e.g. in myocardial infarction, are involved in the regulation of arachidonic acid metabolism was tested. Adrenaline (0.1 micrograms/kg per min for 45 min and thereafter 0.2 micrograms/kg per min for 15 min) was infused to healthy male volunteers to mimic relationships between high levels of adrenaline and arachidonic acid metabolism in myocardial infarction. Adrenaline infusion increased Ca ionophore A23187-induced TXB2 formation in whole blood. The effect was smaller when spontaneous clotting was used as a stimulus. Urinary 11-dehydro-TXB2 excretion, an indicator of total in vivo thromboxane synthesis, increased twofold. Adrenaline infusion decreased both LTB4 and LTE4 synthesis in A23187-stimulated whole blood. These results demonstrate that high levels of adrenaline influence the cyclooxygenase and 5-lipoxygenase pathways of arachidonic acid metabolism differentially in man.