It is shown that in rats exposed to single external irradiation in a dose of 0.5 Gy the concentration of spermatozoa surpasses that of nonirradiated controls. The level of vitamin B 2 increases in the liver and testes of irradiated animals. For the given dose of ionizing radiation, the participation of angiotensin-converting enzyme in cell maturation during spermatogenesis is not observed. Various changes in the content of antioxidant vitamins at different stages of the experiment are described. Key Words: spermatozoa; irradiation; angiotensin-converting enzyme; vitamins; antioxidantsRadiation produces a variety of biological effects on mammals and men. Numerous studies demonstrated a dose-dependent effect of external irradiation on all systems of the organism. However, little is known on the mechanisms of this effect. Organism's response to irradiation include the repair processes. The efficiency of these processes has been little studied [8].The reproductive system, gonads, and cells of spermatogenesis are very sensitive to radiation. We previously described [3,4] the effect of irradiation on the reproductive system in Chernobyl cleanup workers. Analysis of spermograms revealed a rise of morphologically abnormal spermatozoa and the presence of numerous immature cells in ejaculate. At the same time, we found elevated activity of angiotensin converting enzyme (ACE) in spermatozoa, however, this phenomenon has not been explained. Taking into account that the Chernobyl workers were examined on average 4 years after irradiation (the mean dose in the group was 0.158 Gy), we decided to undertake an experimental study for evaluation of immediate effects of various doses of radiation. In the present study we explore changes in the intensity of spermatogenesis, ACE activity in spermatozoa, and the content of vitamins A, E, B 2, and B 6 in the testes, serum, and liver of mature rats exposed to external irradiation in a dose of 0.5 Gy. MATERIALS AND METHODSIrradiation was performed using an IGUR apparatus (Institute of Biophysics, Russian Academy of Medical Sciences): ~37Co emitter, 2.53 rad/sec power, 0.5 Gy (50 rad) dose, and 18 sec exposure. Age-matched animals served as the control.ACE activity was measured as described elsewhere [3]. Spermatozoa were counted as follows: epididymis homogenate in 10 ml physiological solution was centrifuged at 1500 rpm for 20 min, and the resultant sup.ernatant was centrifuged at 2000 rpm for 30 min. The sediment was resuspended in 1 ml buffer (pH 7.4), and the cells were counted in a Goryaev chamber (x200). In parallel, cells suspended in buffer containing a detergent phenylmethylsulfonyl fluoride were counted. It was shown that detergent did not increase ACE activity.For characterization of vitamin metabolism the following parameters were determined. Vitamin B2: riboflavin content in the testes and liver was mea-

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