A comparison of intravenous and oral vit. tolerance tests was made with 16 subjects subsisting on the same constant diet in the same environment. Any combination of ascorbic acid, ribo-flavin, and thiamine could be administered without affecting the urinary excretion of other vits. Nicptinamide had no effect on the excretion of the above 3 vits. The extra excretion after intraven. injn. was complete in 4 hrs.; and after oral adm. in 8-10 hrs. Results of the tolerance tests were formulated mathematically; the control subjects, with no test doses, excreted the vits. at a linear rate. Intravenous tests all fitted simple exponential curves; oral tests all fitted somewhat more complicated double exponential curves, which were most conveniently expressed in terms of hyperbolic functions. The above curves were all derivable on theoretical grounds from consideration of the blood levels as affected by metabolism and renal clearance, and, in the case of oral tests, intestinal absorption. For routine surveys, collection of urine for 1 hr. after an intravenous dose, and during the 2d hr. after an oral dose, might yield results directly comparable with those obtained after collection periods of 4 and 10 hrs. respectively.