High temperature generally constrains plant growth and photosynthesis in many regions of the world; however, little is known about how photosynthesis responds to high temperature with regard to different leaf ages. The synchronous changes in gas exchange and chlorophyll fluorescence at three leaf age levels (just fully expanded, mature, and older leaves) of maize (Zea mays L.) were determined at three temperatures (30 degrees C as a control and 36 and 42 degrees C as the higher temperatures). High temperature significantly decreased the net CO2 assimilation rate (A), stomatal conductance (g(s)), maximal efficiency of photosystem II (PSII) photochemistry (F-v/F-m), efficiency of excitation energy capture by open PSII reaction centers (F'(V)/F'(m)), photochemical quenching of variable chlorophyll fluorescence (q (P)), and the electron transport rate (ETR), whereas minimal fluorescence yield (F (0)) and nonphotochemical quenching of variable chlorophyll fluorescence (q(N)) were increased. The youngest fully expanded leaves had higher A, ETR, and q(P) compared with older leaves. Higher temperature with old leaves led to significant malondialdehyde (MDA) accumulation, a proxy for lipid peroxidation damage from active oxygen species (AOS). MDA content was significantly negatively correlated with A, F-v/F-m, F'(V)/F'(m), and q(P). Thus, the results suggest that photosynthetic potentials, including stomatal regulation and PSII activity, may be restricted at high temperature, together with increasing cell peroxidation, which may be closely associated with leaf age.