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The effects of enhanced UV-B radiation on the gas exchange rates, chlorophyll fluorescence, pigmentation, soluble sugars and carboxylation enzymes of eight maize (Zea mays L.) cultivars grown under Mediterranean field conditions were studied. Supplementary UV-B radiation reduced photosynthetic capacity as a consequence of damage to PSII, increased stomatal resistance, lowered RUBISCO and PEPcase activities and reduced chlorophyll and carotenoid content. Chlorophyll a levels decreased to a greater extent than those of chlorophyll b. Transpiration rate and water-use efficiency were also decreased by UV-B. The accumulation of UV-B absorbing compounds was not stimulated by ultraviolet radiation. The underlying mechanisms for these results are discussed. Apart from these general behaviour patterns, considerable genotypic differences exist within maize cultivars. Further long-term field studies are necessary to assess the interaction between UV-B exposure and climate variables, as well as with agronomic practices.