The rate of photosynthesis was examined in whole leaves of barley infected with brown rust (Puccinia hordei Otth.) and within localized regions of the infected leaf, within brown rust pustules and in regions between them, from flecking to green island formation. In addition, oxygen evolution was measured from localized regions of a rusted leaf after feeding with inorganic phosphate (Pi) to investigate the hypothesis that fungal sequestration of Pi from the host cytoplasm may, totally or partially, be responsible for the decline in photosynthesis. The rate of net photosynthesis and the quantum yield of oxygen evolution declined in whole infected leaves as the disease progressed. However, the rate of gross photosynthesis (per unit chlorophyll) was increased in rusted leaves. Autoradiographic and oxygen evolution studies showed that the decline in net photosynthesis could be attributed largely to a reduction in the rate of photosynthesis in the regions between pustules. Within areas of the leaf invaded by the fungus, gross photosynthesis was increased in comparison to control tissue. Feeding leaf tissue with Pi did not significantly increase the rate of photosynthesis in pustules or regions between pustules, implying that Pi was not previously limiting the rate of photosynthesis. Possible mechanisms underlying the increase and decrease in photosynthesis in different regions of an infected leaf are discussed.