The aim of the trial was to investigate the time course of changes in content of proline and organic compounds affecting sensory quality of ripe red tomatoes after nutrient concentration had been changed. Tomato (Lycopersicon esculentum Mill. cv. Counter) plants were grown in nutrient solutions containing equal nutrient ratios at either low or high concentration (N1=standard solution; N2=5.5 times the standard concentration). Immediately after first harvest of ripe tomatoes, half of the plants of each treatment were transferred to nutrient solutions of higher (from N1 to N2) or lower (from N2 to N1) nutrient concentration. Proline content in leaves of plants transferred to higher or lower nutrient concentration adjusted to control levels (N2 and N1, respectively) within 2 weeks. Growing and ripe fruit reached the corresponding control levels within the same time or 1-5 weeks later. Similar time courses were observed for sugar concentration and titratable acidity in fruit. Apart from leaves, proline content of young growing fruit also increased with increasing radiation intensity at high nutrient concentration. However, in ripe fruit, proline content increased only with low to moderate radiation intensity and decreased when a certain stress level was exceeded. Similar results were found for osmolality and sugar concentration of ripe fruit, while titratable acidity remained rather unaffected by radiation. The role of proline as an indicator of sensory fruit quality is discussed.