“…We have assumed that a dynamic equilibrium exists between the plasma and tissue pools and that the serum vitamin B,, concentration measures all vitamin B,, pools. O n the other hand, it has been suggested that the turnover of vitamin B,, is best described as an open system, which includes one or more compartments with slow turnover rates, and that in such a system equilibrium will not be attained in a finite time (10,11). In seruni, which is considered to be the transport pool, the turnover rate is rapid.…”