  Measurements of the superconducting transition temperature Tc have been
performed by the inductive method on MgB2 at pressures up to 28 GPa. Tc
decreases with applied pressure, exhibiting a cusp at about 9 GPa. We interpret
the appearance of this anomaly in the Tc(P) dependence as a result of the
pressure-induced electronic transition. Recent band-structure calculations for
MgB2 give some support for such an explanation.
