The progress of senescence, under different conditions, of the first leaf of barley uninfected and infected with Erysiphe graminis DC. was determined by measurement of chlorophyll, α-amino-nitrogen and protein-nitrogen content. Uninfected leaves excised and kept in the dark showed a rapid loss of chlorophyll and protein-nitrogen coupled with an increase in α-amino-nitrogen content. Infection by E. graminis did not significantly alter this pattern. However, while exposure to light delayed production of ammonia and loss of chlorophyll and protein-nitrogen in uninfected leaves, it did not do so infected leaves. On whole plants senescence of uninfected leaves was brought about by enclosing them in foil envelopes. Chlorophyll and protein-nitrogen loss was demonstrated but no increase in α-amino-nitrogen content occurred. Infection of first leaves on whole plants in the light led to senescence of affected leaves accompanied by an increase in α-amino-nitrogen and a decrease in chlorophyll content while protein-nitrogen content was unaltered. In leaves both infected and enclosed in foil, protein-nitrogen and chlorophyll content decreased while α-amino-nitrogen content increased more rapidly than in the light. © 1979.