Urinary endogenous oxalate was increased by feeding vitamin B6-deficient or control rats with 5.2% hydroxyproline, or 3% glycine plus 5.2% hydroxyproline. The activities of liver lactic dehydrogenase (LDH), glucose-6-phosphate dehydrogenase (G6PD) , malic enzyme (ME), and ATP citrate lyase were decreased in vitamin B6-DEFICIENT RATS, AND THEIR LIVEr G6PD was further decreased by the addition of glycine and hydroxyproline to their diets. Supplementing control diets with the two amino acids decreased the activities of rat liver LDH, G6PD, and ATP citrate lyase. The effects of glycine and hydroxyproline feeding on the enzymes studied did not appear related to alterations in insulin availability. Since in vitamin B6-deficient rats, there are increases in urinary levels of oxalic and glycolic acids, and glycine, and increases in tissue levels of glyoxylic acid and glycine, the effects of these metabolites on the activities of the above mentioned enzymes were measured. Oxalic acid inhibited the activities of LDH, G6PD, and ME. Glyoxylic acid inhibited LDH and ME, but not G6PD. Glycolic acid inhibited G6PD and ME, but not LDH. ATP citrate lyase was not affected by these substances. Glycine had no effect on the enzymes studied. Diets which increased oxalate excretion generally reduced or did not alter liver and kidney levels of oxalate, glycolate, and glyoxylate. However, the feeding of glycine and hydroxyproline increased kidney oxalate, and liver and kidney glyoxylate in vitamin B6-deficient rats.