We have considered the partitioning of tubulin between monomer and polymer in the cell under conditions of dynamic instability. Dynamic instability adds to the on and off rate constant of steady-state dynamics' new parameters: (1) the rate at which growing microtubules transit to a shrinking phase; and (2) the rate at which shrinking microtubules transit to the growing phase. Under these conditions the free-monomer concentration in the cell increases with total tubulin if the number of nucleating sites is fixed. If the number of nucleating sites increases at fixed total tubulin, subunits shift from the monomer to the polymer phase. These important properties deviate from the traditional equilibrium and steady-state theories and have important implications for the biosynthetic regulation of tubulin.