Autoradiography showed that accumulation of label by hyphae of Peronospora viciae (Berk.) Casp., in leaflets of Pisum sativum L. exposed to 14CO2, was up to 4.5 times greater in the susceptible cv. Krupp Pelushka than the resistant cv. Early Onward. Concentrations of label were similar in apoplastic solutions from both cvs, although a greater volume was available in Early Onward. Uninfected Krupp Pelushka assimilated more label from 14CO2 than Early Onward; para-chloro-mercuribenzene sulphonic acid (PCMBS) treatment and infection reduced assimilation by Krupp Pelushka but not Early Onward. Infected leaf tissue from Early Onward assimilated more label than Krupp Pelushka from [14C]sucrose supplied to the apoplast. Transient treatment with PCMBS reduced assimilation of [14C]sucrose into infected plant tissue to 30 and 61 % of untreated levels, in Early Onward and Krupp Pelushka respectively. In contrast, autoradiography of hyphal fragments showed that the fungus accumulated 1.4 times more label from [14C]sucrose than without PCMBS. The different nutrient status of leaves of Early Onward and Krupp Pelushka, and evidence that label from sucrose is accumulated by intercellular hyphae directly from the apoplast, are discussed in relation to the role of haustoria and mechanisms of compatibility.