A comparison between blood and thromboses is made on behalf of T1 and T2 relaxation data. Thereby, the time interval between the withdrawal and the measurement of blood forms a parameter of investigation. Among the thromboses the surgically removed ones are compared with those aggregated in blood during course of time. As the most outstanding result one can see that the formation of thromboses changes significantly the T1 and T2 relaxation times of 1H und 23Na. The values of the relaxation times obtained for those thromboses which have aggregated within the blood are thereby comparable with those relaxation times found for the surgically removed ones.