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 .