The urinary excretion oTIipoic acid in 2 hours in the early morning in healthy and sick children was determined using microbioassay of Streptococcus faecalis R. The urinary excretion of lipoic acid in healthy children correlated significantly with the gain of the body weight. Urinary excretion of the acid in healthy children was 25 to 45 mg/kg body weight in 2 hours in the early morning. The urinary excretion of lipoic acid in the patients suffering from renal diseases, such as acute gomerulonephritis or nephrotic syndrome, was decreased and the amount of excretion in 2 hours in the early morning was 15 mg/kg body weight on the average. The decrease returned to normal with clinical restoration. In the case of malnutrition due to several causes, the tendency of increase in urinary excretion of lipoic acid was observed. Percentage urinary excretion of lipoic acid in 2 hours following intravenous injection of 1 mg [alpha]-lipoic acid/kg body weight ranged from 0.2 to 0.7% and decreased in renal diseases.