Using 1-(4-trimethylammoniumphenyl)-6-phenyl-1,3,5,hexatriene (TMA-DPH) as fluorescent probe, we have studied platelet membrane fluidity and thrombin induced exocytosis in ten obese children and fifteen controls. Our results indicate a decrease of membrane fluidity, as shown by an increase of fluorescence anisotropy, in platelets from obese patients in comparison with the controls. Associated with the changes in membrane fluidity, platelets of obese subjects showed a decreased sensitivity to thrombin stimulation.