  Mass shifts $\Delta m$ of particles in nuclear matter relative to their
vacuum values are considered. A general formula relating $\Delta m(E)$ ($E$ is
the particle energy) to the real part of the forward particle-nucleon
scattering amplitude ${\rm Re} f(E)$ is presented and its applicability domain
is formulated. The $\rho$-meson mass shift in nuclear matter is calculated at
$2\la E_{\rho}\la 7$ GeV for transversally and longitudinally polarized
$\rho$-mesons with the results: $\Delta m_{\rho}^T \sim 50$ MeV and $\Delta
m_{\rho}^L \sim 10$ MeV at normal nuclear density.
