Oral tryptophan loading tests were performed in a patient with photosensitive pellagra-like skin rash and cerebellar ataxia but without hyperaminoaciduria. Plasma tryptophan concentrations after loading were similar in the patient and control subjects. Average urinary excretion of tryptophan in the patient from 0 to 6 and 6 to 12 hr was 2.69 and 2.58 mumol/kg, respectively; that in the control subjects was 0.82 and 0.34 mumol/kg, respectively. However, the average renal clearance of tryptophan during the first 6 hr of the loading tests in the patient was 0.757 ml plasma/1.73 m2 and that in the control subjects was 0.706 ml plasma/1.73 m2. Renal excretion of kynurenine in the patient was markedly decreased. The average from 0 to 6 and 6 to 12 hr in the patient was 1.90 and 1.13 mumol/kg, respectively; that in the control subjects was 12.90 and 18.15 mumol/kg, respectively. Under ultraviolet light, paper chromatograms of urine from the patient showed a deficiency of xanthurenic acid, kynurenic acid, kynurenine, and formylkynurenine. The deficiency of formylkynurenine in the patient's urine was confirmed by staining the paper chromatograms with Ehrlich's reagent. The patient was "sensitive" to oral nicotinic acid treatment; however, oral nicotinamide was well tolerated with improvement in the photosensitive skin rash.