Rats were subjected to whole-body irradiation with doses of 5.0 Gy and 10.0 Gy. After intervals of 1.5 hours, 3, 6, 12 and 24 hours following irradiation the authors tested the aggregation capacity of thrombocytes after induction with adenosine--5--diphosphate and the cAMP level in isolated thrombocytes. Statistically significant differences between experimental and control groups were found particularly at some time intervals in rats irradiated with 10.0 Gy. The authors found a significant acceleration of the time of maximal aggregation at time intervals of 3.6 and 24 hours following irradiation with 10.0 Gy. Only at the time interval of 3 hours after irradiation with 10 Gy a statistically significant reduction of the maximum aggregation, as compared with the control group, occurred, and similarly the tang alpha value indicates that also the initial rate of aggregation was lower. The cAMP level in isolated thrombocytes was markedly reduced in particular after 5.0 Gy already during the early intervals after irradiation. A temporary increase of the cAMP levels was recorded 3 hours after irradiation with 10.0 Gy which may explain the reduced aggregation capacity of thrombocytes found during the same time interval. The results of both tests are not sufficient evidence of a hypercoagulation state after irradiation with the mentioned doses within 24 hours after irradiation.