It was conceivable that the previously reported elevated pentose-shunt activity in human rheumatoid synoviocytes could be at the expense of glycolytic activity. To test this possibility the activities of glyceraldehyde 3-phosphate and lactate dehydrogenase, the two dehydrogenase enzymes of the latter pathway, have been investigated in the synovial lining cells in fresh sections of nonrheumatoid and rheumatoid synovial tissue. To measure the activity solely in the lining cells, apart from that in underlying infiltrating cells, quantitative cytochemical reactions have been used; the activities were measured by microdensitometry. The results showed highly and significantly increased activity of both enzymes in the rheumatoid cells. Increased activity was also found in synoviocytes in nonrheumatoid synovial tissue after trauma, so that the increased activity of these enzymes is not characteristic of the rheumatoid condition. However, the results indicate that the increased pentose shunt activity in rheumatoid synovial lining cells is not at the expense of glycolytic activity but may be part of an enhanced potential for utilising glucose 6-phosphate in these cells.