Male weanling rats were maintained on a thiamine-deficient diet for 4 weeks, and compared with ad libitum and pair-fed controls. Thiamine deficiency led to slow growth and finally a decrease in body weight. Liver and kidney weights of the deficient rats were low, but appropriate to the body weight. Thiamine deficiency also caused a significant decrease in erythrocyte transketolase levels. The decarboxylation of glyoxylate both via the glyoxylate oxidation cycle and α-ketoglutarate:glyoxylate (α-KG:GA) carboligase was significantly lower in the liver and kidney mitochondria, leading to accumulation of glyoxylate in the tissues and its excretion in the urine. Part of the accumulated glyoxylate is converted to oxalate, causing hyperoxaluria.