Levels of malate dehydrogenase (MDH), lactate dehydrogenase (LDH) and their isoenzymes in tears of normal Chinese subjects and patients with ocular surface disorders were determined. The normal values of tear LDH and MDH were found to be 45.51 +/- 23.00-81.35 +/- 37.84 mumol.s-1/L and 11.00 +/- 5.33-19.50 +/- 9.17 mumol.s-1/L respectively, disregarding sex or eye distinction. The LDH/MDH ratio reflected sensitively the metabolism of corneal and conjunctival epithelium. The MDH isoenzymes comprised MDHs and MDHm, the former accounting for 80.0%-89.1%. The LDH isoenzymes comprised 5 varieties, of which the ratio H/M of subunit H to subunit M was 0.196 +/- 0.02. The changes in LDH isoenzymes were helpful to the differential diagnosis of external eye diseases, and the increase of MDHm reflected sensitively the degree of injury to the corneal epithelium.