Paclobutrazol increased root to shoot ratio, levels of chlorophyll and carotenoid pigments and accelerated chlorophyll formation in etiolated maize seedlings (Zea mays L. cv. Pioneer 3902 and Pride 5). Treated leaves had more epicuticular wax, were thicker and wider because of enlarged vascular bundles and all cells. Chloroplasts were larger with more stromal lamellae and fewer, but larger, grana stacks. Paclobutrazol increased survival rate following chilling and alleviated damage symptoms from chilling and exposure to paraquat in Pioneer and Pride, the degree of protection being greater in Pride (the less stress tolerant cultivar). During photoinhibitory conditions, dissociation of grana, decreased Fv:Fm ratio and ion leakage occurred earlier and at a faster rate in control leaves. Leaves of treated plants had higher violaxanthin and lutein levels and there was less interconversion of violaxanthin to zeaxanthin. Paclobutrazol-induced stress protection in maize seedlings was associated with structural changes in the chloroplast.