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The first part of thesis is engaged in explanation of some basic concepts regarding an elected topic: radioactivity, isotope, nuclide, halftime, gamaspectrometry etc. In the second part I deal with element cesium and influence of the Chernobyl accident. The final part processes questions of cesium - 137 in wood ecosystem. It is mainly concerned with research and gamaspectrometric measurement of wood substance, bark and samples of soil. In practical part (it is one part of the final part) I made taking of samples (especially soil from the depth 0-5 cm, soil from the depth 5 - 20 cm, wood substance and forest planting) from some localities above all the South Bohemian Region and next their gamaspectrometric measurement (quantitative analysis of cesium - 137 in wood ecosystem) at SÚJB in České Budějovice according to methodology.Measured values of cesium - 137 in samples were relatively low and its impact on the health of our population is in regard to other sources of ionizing radiance insignificant. The supposed hypothesis was herewith confirmed.

At the nuclear power plant accident, the radionuclides with very short half-life come to enviroment, they decay the period from several second to several months. On the other sides come to enviroment radionuclides with long half-life. Their radionuclides occur in surrounding of the power plant for hundreds of years before they decay totally. Immediately after a nuclear power plant accident, radioiodine represents the most severe health hazard for population. Because it is a radionuclide with short half-life, it causes the danger during first months after the accident. Its health hazard lies in the fact, that it gets to food chains and irradiates a man, who consumed contaminated water and food. Radiocaesium and radiostrontiom are the next important radionuclides. They have long half-lifes, it means that will decay after hundred years after the accident and they endanger population a long time after the accident yet.