Exchanges of CO2 and water vapour were examined in groundsel, Senecio vulgaris, grown and infected with rust, Puccinia lagenophorae, under laboratory conditions. The effects of drought were examined by withholding water from plants from 2 days until approximately 10 days after infection, after which further daily water losses were restored. Net photosynthesis was inhibited and dark respiration was stimulated in rust-infected leaves but, in young healthy leaves on the same plant, gross and net photosynthesis were stimulated and photorespiration was inhibited. The growth of plants was stunted 15 days after infection because the growth of leaf area was inhibited and, thereby, the photosynthetic capacity of the plant was reduced. Drought inhibited the growth of leaf area in both healthy and infected plants and, when combined with rust, had additive deleterious effects on net photosynthesis per plant and on growth. After sporulation, rusted plants transpired more rapidly than did healthy controls and leaf water potential fell to reach - 0.95 and-1.92 MPa in well-watered and drought-stressed plants respectively, compared with -0.18 and - 1.2 MPa in healthy controls, 20 days after infection. Rust impaired the normal increase in Water-Use Efficiency in response to drought.