Changes in the capacities of enzymes in various metabolic pathways have been measured during infection of cotyledons of Cucurbita pepo L. with cucumber mosaic virus (CMV). Starch accumulation and low sucrose content, which are characteristic of the early stages of infection, are reversed in the later stages of infection. The decline in starch correlated with a reduced capacity for starch synthesis (ADP-glucose pyrophosphorylase) and a rise in the capacity for starch degradation (total starch hydrolase, starch phosphorylase). 14CO2 feeding experiments, conducted at saturating CO2 concentration, show that the newly-assimilated carbon was lost at a lower rate from infected cotyledons and less was incorporated into structural carbohydrates, phosphorylated intermediates plus organic acids, more into soluble sugars, amino acids and proteins. At a later stage of infection there were dramatic increases in respiratory capacity and a substantial alteration of carbohydrate metabolism. The infection had a large stimulatory effect on the capacity for oxidative pentose-phosphate pathway (glucose-6-phosphate dehydrogenase, 6-phospho-gluconate dehydrogenase), glycolysis (ATP- and pyrophosphate-dependent phosphofructokinases), tri-carboxylic acid cycle (isocitrate dehydrogenase, fumarate hydratase), anaplerotic reactions (NAD-dependent malic enzyme, phosphoenolpyruvate car-boxylase) and oxidative electron transport (cytochrome c oxidase). While there were no overall changes in photosynthetic rate (measured in saturating CO2), infection either reduced (Rubisco and glycerate kinase) or did not affect (chloroplastic fructose bis-phosphatase and hydroxypyruvate kinase) the capacities of the photosynthetic carbon reduction pathway or the photosynthetic carbon oxidation pathway.