  We discuss the evolution of anisotropic boost-invariant quark-gluon plasma
possibly created at the early stages of relativistic heavy-ion collisions. Our
considerations are based on the recently proposed formalism that is an
extension of the relativistic perfect-fluid hydrodynamics. We analyze i) the
pure partonic system described by the anisotropic phase-space distribution
function and ii) the system of partons interacting with the local magnetic
fields. The second analysis is a simplified attempt to include the effects of
color fields on the particle dynamics. Our model results are discussed in the
context of early thermalization and isotropization. Under general assumptions
of the particle, energy, and momentum conservations we show that for large
evolution times the ratio of the longitudinal and transverse pressures of the
pure partonic system tends to zero. Hence, the system with the initial momentum
distribution elongated along the beam axis always passes through the isotropic
stage where the transverse and longitudinal pressures are equal. The inclusion
of the magnetic field in this case gives negative contribution to the
longitudinal pressure, hence the transient stage when the total longitudinal
and transverse pressures become equal may be reached earlier, depending on the
strength of the field.
