  Dependences of the X-ray luminosity (L$_x$) of single pulsars, due to
ejection of relativistic particles, on electric field intensity, rate of
rotational energy loss (\.{E}), magnetic field, period, and the energy spectra
of the ejected particles are discussed. Influence of the magnetic field and
effects of some other parameters of neutron stars on the L$_x$-\.{E} and the
L$_x$-$\tau$ (characteristic time) dependences are considered. Evolutionary
factors also play an important role in our considerations. We find that only
the pulsars with L$_{2-10keV}$$>10^{33}$ erg/s have pulsar wind nebula around
them. The pulsars from which $\gamma$-ray radiation has been observed have low
X-ray luminosities in general.
