A group of 20 male rats of mean initial body weight of 65.53 g were irradiated for 7 wk (5 days per wk, 4 hr per day) with an electromagnetic pulse field of the following parameters: working frequency 2,736.5 MHz; repeated frequency 395 Hz; pulse width 2.6 musec; vertical polarization; mean power density 24.4 mW/cm2; accuracy of measuring +/- 6%. The rectal temperature of experimental animals increased during irradiation by a maximum of 0.5 degrees C. Blood was taken before irradiation, at the end of the 1st, 3rd, 5th, and 7th wk of irradiation, and at the end of the 1st, 2nd, 6th, and 10th wk after irradiation was completed. The parameters under study included the hematocrit value; number of leukocyte differential count in both absolute and relative proportions; activity of alkaline phosphatase in neutrophil leukocytes; and body weight increase. The results were compared with parallel data obtained from a control group of 20 animals and evaluated by Student's t test at a significance level of 1%. In the second half of the irradiation period the experimental animals exhibited significantly lower mean hematocrit values, lower numbers of leukocytes, and lower absolute numbers of lymphocytes. These changes disappeared gradually within 10 wk after completed irradiation. Activity of alkaline phosphatase in neutrophil leukocytes was significantly increased in the 1st wk of irradiation and dropped transiently after the irradiation. In the post-irradiation interval experimental animals displayed significant decline in rate of body weight increase. The level of the other examined parameters did not differ from the controls.