The activities of human lens aldehyde dehydrogenase, glyceraldehyde-3-P dehydrogenase, polyol dehydrogenase, triosephosphate isomerase and glutathione reductase were measured prior to and following their exposure to oxidation. Active oxygen species were generated by the reaction of either methylene blue or riboflavin with light over a 60 minute time interval. It was found that oxidants generated by both photosensitizers rapidly diminish the activities of glutathione reductase and glyceraldehyde-3-P dehydrogenase but do not alter the activity of triosephosphate isomerase. After an initial time delay, PD activity likewise was abolished and 50% ADH activity remained at the end of the reaction sequence. All enzyme activities affected declined at a faster rate in the presence of methylene blue than riboflavin, and methylene blue itself served as an enzyme inhibitor to the catalytic function of glutathione reductase and glyceraldehyde-3-P dehydrogenase. These findings suggest that singlet oxygen formation not only alters the properties of the crystallin components of the human lens but also damages several of their enzyme associated counterparts.