The response of four hybrids and their lines to water stress was tested in conditions of water culture. Water stress was induced by PEG. Plant reaction to water stress was estimated at the basis of nitrate reductase activity, proline accumulation, transpiration intensity, and relative water content. According to the results obtained it may be concluded as follows. Nitrate reductase activity, free-proline concentration, transpiration intensity and relative water content significantly differed among the tested hybrids and lines. Water stress reduced nitrate reductase activity, transpiration intensity and relative water content, and increased free-proline concentration in all hybrids and lines, although not equally. The greatest change was observed in the concentration of free-proline which increased in all tested hybrids and lines approximately 14 times, while nitrate reductase activity was lower by approximately 50% than that in the control plants. Nitrate reductase activity and free-proline accumulation which suffered great modification in plants under water stress, may be considered as the two parameters which indicate sunflower resistance to drought. However, this should be examined in field conditions, as well. Dependence between the examined morphological parameters and the response of the hybrids to water stress was not discovered.