  We report on results of two BeppoSAX observations of the Z source GX 17+2. In
both cases the source is in the horizontal branch of the colour-intensity
diagram. The persistent continuum can be fit by two-component models consisting
of a blackbody plus a Comptonization spectrum. With one of these models, two
solutions for the blackbody temperature of both the observed and seed photons
for Comptonization are equally accepted by the data. In the first observation,
when the source is on the left part of the horizontal branch, we observe a hard
tail extending up to 120 keV, while in the second observation, when the source
moves towards right in the same branch, the tail is no longer detected. The
hard ($\ga 30$ keV) X-ray emission can be modeled either by a simple power-law
with photon index $\Gamma \sim$ 2.7, or assuming Comptonization of $\sim$ 1 keV
soft photons off a hybrid thermal plus non-thermal electron plasma. The
spectral index of the non-thermal injected electrons is $p \sim 1.7$. The
observation of hard {X-ray} emission only in the left part of the horizontal
branch could be indicative of the presence of a threshold in the accretion rate
above which the hard tail disappears. An emission line at 6.7 keV with
equivalent width $\sim 30$ eV is also found in both observations. We discuss
these results and their physical implications
