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The concentrations of glycolate (hydroxyacetate) and lactate are significantly elevated above control values in urines from streptozotocin-diabetic rats, regardless of whether data are expressed in terms of μg/ml urine or μg/day. The same levels of oxalate and glyoxylate are excreted in 24 h in the urines from normal and diabetic rats. Lactate levels are elevated above control values in serum from streptozotocin-diabetic rats. The elevation of glycolate levels in diabetic rat urine compared to control values occurs regardless of diet and regardless of whether rats were fed or fasted during the 24 h urine collection period. Rat liver glycolate oxidase may be used to assay glycolate concentrations in the presence of up to 500 μg/ml l -lactate when pH 8.6 Tris-Cl is used as buffer. Results obtained with this assay compare qualitatively with the standard colorimetric assay using 2,7-dihydroxynaphthlene for glycolate determination. Beef liver glycolate oxidase is not effective for use in glycolate assays. The identity of urinary glycolate was confirmed by gas-liquid and by paper chromatography.