The responses of human subjects to a low pantothenic acid test diet and to the same diet supplemented with 10 mg pantothenic acid daily for 63 days were observed. Pantothenic acid in urine and blood and also nitrogen balance were used as criteria for nutritional evaluation. The mean daily urinary pantothenic acid excretion decreased from 3.05 to 0.79 mg in male adult subjects fed a pantothenic acid deficient diet and increased from 3.95 to 5.84 mg in 4 subjects fed a 10 mg supplemented diet from the beginning to the end of 63-day study. Subsequently, a test dose of 100 mg of pantothenic acid was given to both groups for a 7-day period. Subjects previously deprived of pantothenic acid retained 63% of the test dose on the first day of the test period in contrast to 48% retained by supplemented subjects. On the 7th day, both groups retained approximately 40% of the dose. While in general, blood pantothenic acid levels decreased in unsupplemented subjects and remained constant in supplemented subjects, blood pantothenic acid responded less readily to intake than urinary pantothenic acid. Nitrogen retention tended to be higher in supplemented subjects than in those deprived of dietary pantothenic acid.