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In plants the chloroplast is a major site of free radical production and paclobutrazol protects plants by increasing antioxidant defense systems. Leaves from paclobutrazol-treated maize (Zea mays L. cv. Pioneer 3902 and Pride 5) had larger chloroplasts and grana stacks, and more stromal lamellae. Treated leaves had more epicuticular wax and were thicker and wider owing to enlarged vascular bundles, epidermal, mesophyll, and bundle sheath cells. Paclobutrazol increased survival rate following chilling (2°C for 8 h) and alleviated damage symptoms owing to chilling and exposure to paraquat (60 and 120 nM) in 'Pioneer' and 'Pride', the degree of protection being greater in 'Pride' (a less stress tolerant cultivar). Violaxanthin and lutein levels were higher in treated etiolated and light-grown plants. During photoinhibitory conditions of 5°C and 1300 µmol m-2 s-1 there was less interconversion of violaxanthin to zeaxanthin in treated leaves. Dissociation of grana, changes in the osmiophillic properties of the thylakoids, decreased Fv/Fm ratio, and ion leakage occurred earlier and at a faster rate in control leaves. Protection from paraquat and photoinhibitory damage in maize leaves by paclobutrazol indicates that treated plants have a more efficient free radical scavenging system.Key words: chloroplast, paclobutrazol, stress, triazole, xanthophyll, Zea mays.