Effects of and recovery from continuous wear of four rigid gas permeable (RGP) contact lenses was assessed by noninvasive measurement of lactate dehydrogenase (LDH) activity and isozyme pattern in rabbit tears. Oxygen transmissibility (Dk/L) of lenses used was 27, 44, 84, and 97 x 10(-9) (cm/s)(ml O2/ml mm Hg); lens thickness (0.15 mm) and diameter (14.0 mm) were standardized. Lenses were worn continuously for 90 days; recovery was assessed 30 days after cessation of lens wear. LDH activity was measured by UV rate assay; isozyme subtypes were determined by agarose gel electrophoresis. Light and scanning electron microscopy (LM, SEM) were used with the determination of total protein as additional measures of lens effects. LDH levels were inversely correlated with lens Dk/L values; low Dk/L values increased the anaerobic (LDH4,5)/aerobic (LDH1,2,3) subtypen ratio indicating in vivo metabolic shift. SEM observations were consistent with these results. There was no significant difference in the total cell content of tears or total tear protein levels between control and RGP test-wear groups. Measurement of tear LDH activity and isozyme ratios appears to provide a sensitive, noninvasive assessment of the effects of RGP lens-induced hypoxia over time on the corneal surface. A level of Dk/L of > or = 84 appears best for maintaining corneal physiology during extended wear. Recovery from chronic lens-induced hypoxia is characterized by a return to normal tear LDH levels and isozyme subtypes.