Monosaccharides, sucrose and acyclic polyols in healthy and infected first leaves of oat and wheat susceptible to crown rust and stem rust respectively were quantitatively determined. In oat tissues heavily infected with crown rust (thirty-four lesions/cm segment), sucrose and reducing sugars increased during flecking and then declined. During sporulation of wheat stem rust-infected tissues, sucrose levels at an infection intensity of eight lesions/leaf were greater than those of comparable healthy tissues whereas, at a higher infection intensity (107 lesions/leaf), there was a slight decline in sucrose with increases in both fructose and glucose. Soluble sugars (combined sucrose, fructose and glucose) were greater in the ‘pustule area’ compared with ‘region adjacent’ on the same leaf of wheat. Activity of acid invertase in infected tissues is related to infection intensity in crown rust of oat. However, in wheat stem rust infected tissues, a stimulation of acid invertase only occurred during the appearance of definite type 4 lesions. At the sites of infection, there were increases in mannitol and arabitol during flecking and sporulation, and a direct correlation between acyclic polyol levels and numbers of erupted lesions/leaf.