  We present high dispersion HST GHRS UV spectroscopic observations of 8 HII
galaxies covering a wide range of metallicities and physical properties. We
have found Lyalpha emission in 4 galaxies with blueshifted absorption features,
leading to PCygni like profiles in 3 of them. In all these objects the OI and S
II absorption lines are also blueshifted with respect to the ionized gas,
indicating that the neutral gas is outflowing in these galaxies with velocities
up to 200 km/s.The rest of the sample shows broad damped Lyalpha absorption
profiles centered at the wavelength corresponding to the redshift of the H II
emitting gas. We therefore find that the velocity structure of the neutral gas
in these galaxies is the driving factor that determines the detectability of
Lyalpha in emission. Relatively small column densities of neutral gas with even
very small dust content would destroy the Lyalpha emission if this gas is
static with respect to the ionized region where Lyalpha photons originate. The
situation changes dramatically when most of the neutral gas is velocity-shifted
with respect to the ionized regions because resonant scattering by neutral
hydrogen will be most efficient at wavelengths shorter than the Lyalpha
emission, allowing the Lyalpha photons to escape (at least partially). This
mechanism complements the effect of porosity in the neutral interstellar medium
discussed by other authors. The anisotropy of these gas flows and their
dependence on the intrinsic properties of the violent star-forming episodes
might explain the apparent lack of correlation between metallicity and the
frequency of occurence and strength of Lyalpha emission in star-forming
galaxies. Attempts to derive the comoving star-formation rate at high redshifts
from Lyalpha emission searches are highly questionable.
