3H‐octopamine was found to be accumulated in human platelets, achieving a maximum concentration gradient of 30:1. Its accumulation was partially inhibited by reserpine, imipramine, serotonin, ouabain, dinitrophenol, and iodoacetate. When octopamine was added to platelet preparations, it led to a decrease of both endogenous and 14C‐serotonin. To determine whether octopamine accumulates in human platelets in vivo, preparations from 6 patients receiving monoamine oxidase inhibitors and 17 control subjects were assayed enzymatically for octopamine. Octopamine was detectable in all of the drug‐treated patients, averaging 0.45 ± 0.06 ng/mg protein, while only 4 of the 17 control subjects had detectable (>0.05 ng/mg protein) platelet octopamine. Although much lower than platelet serotonin levels, these octopamine levels are in the range of those reported for platelet norepinephrine and epinephrine.