During 1957-1962 the routes and rates of B,, excretion were studied by faecal and whole-body counter techniques, and long-term studies of plasma clearance performed. In the present analyais, the equations of the experimental curves were obtained with an analogue computer, and the simplest compatible model selected. The equations were solved mathematically for a particular model and hence, using Berman and Schoenfeld's method, all other possible models were calculated. Finally, the time curves for radioactivities in experimentally inaccessible pools were generated with the computer. Three pools were required, one apparently intracellular (containing over 99% of the body-B,,), one extracellular B,,, and the third unidentified. In this model, therefore, B,, is assumed to have n t least an approximately uniform turnover rate in most of the intracellular pool. The present method can be used to measure total body content of B,,, which was found t o be 3.03 mg in the subjects studied. The physiological loss of B,, was calculated to be 1.2 pg/day, and the relations of this figure to B,, requirements in man were discussed. Preliminary results in pernicious anaemia indicate a decreased cellular uptake of Bls.
