Washed human platelets incubated in different concentrations of fibrinogen were activated by thrombin and aggregated in the presence of Ca2+or did not aggregate when EDTA was present. They were analyzed by transmission electron microscopy and computer-aided three-dimensional reconstruction. The volumes and surface areas of the reconstructed models were calculated. The quotients of the values calculated for the whole platelet and the surface-connected canalicular system were taken as measures of the degree of surface invagination. Increasing the concentrations of fibrinogen reduced the values of the quotients indicating enhanced internalization of surface membranes, and tended to smoothen the outer surfaces to obtain spherical shapes. The invaginations are much more pronounced in platelets that did not aggregate in the presence of EDTA suggesting that aggregation fixes some membrane areas that otherwise would be redistributed to the platelet's inner compartments.