The effect of leaf age on the resistance of the plant photosynthetic apparatus (PSA) to high cadmium content (100 μm) in the root medium was studied in a laboratory experiment with barley (Hordeum vulgare L.) as the study object. After 4 days of exposure to the cadmium-bearing solution, a young leaf, where the metal concentration was the highest, demonstrated a decline in green pigment content, reduced electron transport rate and photochemical quenching of chlorophyll fluorescence, but the rate of photosynthesis remained at the control level owing to protective mechanisms. In a mature leaf, where cadmium content was the lowest, no inhibiting effect of the metal on PSA was observed, and the rate of photosynthesis increased. In an ageing leaf, where the metal content was quite high, most of the investigated PSA parameters declined and photosynthesis slowed down. We conclude that the distinctions between leaves of different age in the way their PSA responded to cadmium were related to the metal content in them and to the structural and functional modifications that take place in the cells and tissues during the leaf ontogeny. Presumably, this response is a mechanism of the plant metal resistance which involves the whole plant.