Maize (Zea Mays L.) is one of the most important cultures in the world along wheat and rice. As a tropical plant, maize is sensitive on frost and water deficiency, and according to needs for water, it is one of plants that economical spents water. Losses caused by the water deficit stress in total crop production on the Earth (agriculture, forestry) are greater than losses caused by all other biotic and abiotic factors together. The objective of this master thesis is to determine the effect of water deficit stress on the photosynthetic activity and oxidative stress of maize. In the experiment was used hybrid Pajdaš, that is one of the most usually commercial hybrids in agricultural production in Croatia. Experiment was set in growth chambers in Faculty of Agriculture in Zagreb, on scheme of random blocked schedule in three repetition. Treatments of drought was applied in stage of three developed leaves,and plants were irrigated with polyethylene glycol dilutions (in concentrations that caused osmotic potential of the media from -5 and -1 MPa (control). Each treatment was applied in a period of three weeks, but measurements of chlorophyll concentration index (CCI), and parameters of photosynthesis (photosynthesis intensity (A), stomatal cunductance (gs), transpiration intensity (E) and intercelular concentration od CO2 (ci), were practised 24 hours after treatment application. In the end of the experiment is determined concentration of proline, the degree of oxidation of lipid and leakage of ions. Drought stress was not caused falling in chlorophyll content, but has caused significant reduction of parameters of photosynthesis (A, E, gs, ci) and we can assume that falling of photosynthetic intensity isn't caused with reduce capability of the photosynthetic apparatus for absorption of light. The main reason of reducing photosynthesis intensity is reduction of stomatal conductance. By reduction of stomatal conductance, at the same time it is come to the reduction of intercelular concentration of CO₂, which caused falling of photosynthesis intensity. Concentration of proline, leakage ions and concentration of malondialdehyde (MDA) was bigger in the treatment of drought (-5 MPa), compared to control. Increased lipid peroxidation caused distortion of the structure of biomembranes which has caused increased leakage of ions from cells, which is possible cause of falling of the photosynthesis intensity at prolonged exposure of plants to drought stress.