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The role of microtubules in adhesion and in the adhesion-stimulated release reaction of platelets from African green monkeys has been studied using conventional (100 kV) and high voltage (1000 kV) stereo electron microscopy. Upon exposure in vitro to either glass or a carbon-stabilized formvar surface, platelets were rapidly activated and extended numerous filopodia. The adhesion process evolved over a period of 10–20 min with extension of a delicate hyalomere across the surface between adjacent filopodia. Microtubules, which form a circumferential ring in the resting cell, were dissociated upon cell activation and then reassembled in patterns which conformed to the general cell shape. In dendritic cells with numerous filopodia the microtubules were oriented radially. With subsequent hyalomere development, the microtubules paralleled major cell axes. This morphological transition of platelet adhesion was accompanied by the release of β-thromboglobulin (B-TG) which was maximal at 20 min, a time that corresponded to maximal hyalomere extension. Treatment of platelets with colchicine or vinblastine sulfate prior to adhesion dissociated the microtubules, but affected neither the morphological transition nor β-TG release. Our observations separate microtubules from adhesion-related events and suggest that during adhesion the microtubules, by conforming to cell shape, may simply provide cytoskeletal support.