  The pair-breaking critical current density j_{d} of magnesium diboride was
determined over its entire temperature range by a pulsed dc transport
measurement. At fixed low values of current density j, the resistive transition
temperature Tc shifts in the classic \Delta T_{C}(j)/T_{C}(0) \propto
-[j/j_{d}(0)]^{2/3} manner, with a projected j_{d}(0) \approx 2 \times 10^{7}$
A/cm^{2}. The directly measured j_{d}(0), from current-voltage ($I$-$V$)curves
at different fixed temperatures, has a similar value and the overall
temperature dependence j_{d}(T) and magnitude are consistent with
Ginzburg-Landau theory. Keywords: pair, breaking, depairing, superconductor,
superconductivity, flux, fluxon, vortex, mgb2
