The effect of glycine administration on acute leucine loading (125 mg/kg) was tested in a patient with isovaleric acidemia. Serum isovaleric acid at 1-3/4 hr after the leucine loading alone was elevated to 5.60 mg/100 ml and urinary isovaleryglycine excretion was 9.90 mg/mg creatine/24 hr. Whe the same amount of leucine was given with glycine (250 mg/kg) serum isovaleric acid was only 0.93 mg/200 ml. Unfortunately, urine was collected for only 12 hr after the leucine-glycine loading. However, the amount of urinary isovaleryglycine was 26.2 mg/mg creatine in this period. In the following experiments in which a meal containing 80 mg leucine/kg was given, serum isovaleric acid was elevated to 1.14 and 1.01 mg/100 ml at 3 hr and 6 hr after the loading, respectively. How-ever, serum isovaleric acid was only 0.53 and 0.79 mg/100 ml at 3 and 6 hr, respectively, when the identical mean was given with 2 g glycine. The effect of long term glycine administration (250 mg/kg/24 hr) was also tested. It did not prevent two ketotic episodes which were caused by infections. However, the duration of clinical symptoms such as vomiting and a large anion gap in the acute episodes were much shorter with rectal glycine administration. The patient's linear growth and weight gain durin glycine administration was much better than that in the pretreatment period.