Translocation of 14C from first leaves of barley is reduced by rust infection. Infection of the leaf base alone results in enhanced loss of 14C from the leaf tip, whilst infected leaf tips show greatly reduced loss. Concentrations of soluble sugar are lower and those of storage polysaccharide higher in rusted leaves. Their amounts and the kinetics of flux of 14C through them are used to validate a compartmental model of translocatory carbon flux derived from the pattern of 14C-effiux from the leaf. This model suggests that the major consequences of infection are reduced size of the sucrose transport pool and elimination of a phase of efflux with a half-life of 2h.