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The uptake and storage of 5-hydroxytryptamine by human platelets from normal donors and patients with the carcinoid syndrome are reported. When platelets were incubated in the presence of 3H-5-hydroxytryptamine (50 microM), an initial phase of rapid uptake was observed until 5-hydroxytryptamine levels of 1--2 microgram/10(9) cells were reached. Further accumulation occurred more slowly. Electron microscopic autoradiography of cells after 5-hydroxytryptamine uptake for 60 minutes revealed that in platelets from normal donors most of the label was localized in membranes (52%), alpha granules (27%), and cytoplasm (4%). Dense bodies were associated with 18% of the silver grains. Similar membrane (50%) and alpha-granule (27%) labeling was found with platelets from patients with carcinoid syndrome. However, more cytoplasmic labeling (10%) and less dense-body labeling (11%) were found with these cells. The results support a hypothesis of two or more sites for storage of 5-hydroxytryptamine in mammalian platelets and suggest that minimal exchange between cytoplasmic and dense-body amines occurs in carcinoid platelets.