An experimental study on effects of bulunt trauma on immune system and thymus of rats Fifty albino rats, aged between 45 and 55 days, were used in this study. The animals were divided into five groups, ten rats in each group. The animals in group I were kil¬led without any operation. Sham-operated groups II and III were killed after 2 and 5 days of postoperative abservation, respectively. The animals in groups IV and V were traumatized by a blunt percussion device that was left in a hole 3 mm in diameter of cranium which were surgically prepared. The animals in the t traumatized groups IV and V were killed after 2 and 5 days of postoperative observation, respectively. For sacrifice, the rats was deeply intraperitoneally anesthetized with ketamin in a dose of 0.1mg/g of body weight, and the blood was taken into a tube. After that intracardiac perfusion was carried out, with isotonic saline and 19% formalin (20 to 30 ml). Follo¬wing perfusion, tha thymus was removed and weighted.The thymus tissues were pre¬pared for histopathologic analysis. The number of lymphocytes and concentration of cortisol in the blood were calculated for each group. Mann-Whitney U test is used to test for significance between experimental and control groups. There was no significant difference among tha groups in the weights of thymus, however there was a significant decrease in the numbers of lymphocyte and the concentrations of cortisole between group I and group V (p