The experiment was carried out during 2008-2009, using the method of water cultures. The biological material was head lettuce var. ‘Nochowska’. The first experimental factor was concentration of sodium chloride in the medium: the control, 68 and 136 mÌ NaCl, whereas the second factor was the level of fertilization with potassium: + 20%K, the control (without additional fertilization with potassium). The control was a full Hoagland’s medium (pH 5.5). A significant decrease in the intensity of CO2 was observed in plants grown in the medium of 136 mÌ NaCl concentration, with no additional nutrition with potassium. An increase in salinity, in the hydroponic growing media enriched with 20% of potassium, did not cause a decrease in photosynthesis. A decrease in the intensity of transpiration and an increase in the index of water use effectiveness in photosynthesis were recorded along with an increase in the NaCl concentration in plants additionally nourished with potassium.The addition of 20% of potassium caused, independently of the level of salinity of the media, an increase in the content of all the forms of chlorophyll in the leaves of the studied plant.