The effects of desiccation on the activity of several enzymes from Acrocladium cuspidatum were investigated; with the exception of glyceraldehyde 3-phosphate dehydrogenase prolonged desiccation had little effect on the enzymes studied. The response of the NADP+-dependent glyceraldehyde 3-phosphate dehydrogenase to sulphydryl compounds changes during desiccation; when extracted from desiccated shoots it showed a six to twenty times stimulation of activity in the presence of reduced glutathione, that from fresh shoots showed only a twofold stimulation. The effects of glutathione on the enzyme's activity were examined in species with varying resistance to desiccation. It is suggested that the decline in photosynthesis during desiccation may be explained in part by the oxidation of sulphydryl residues essential for the catalytic activity of the NADP+-glyceraldehyde 3-phosphate dehydrogenase.