  The fractionation of nitrogen (N) in star-forming regions is a poorly
understood process. To put more stringent observational constraints on the
N-fractionation, we have observed with the IRAM-30m telescope a large sample of
66 cores in massive star-forming regions. We targeted the (1-0) rotational
transition of HN$^{13}$C, HC$^{15}$N, H$^{13}$CN and HC$^{15}$N, and derived
the $^{14}$N/$^{15}$N ratio for both HCN and HNC. We have completed this sample
with that already observed by Colzi et al. (2018), and thus analysed a total
sample of 87 sources. The $^{14}$N/$^{15}$N ratios are distributed around the
Proto-Solar Nebula value with a lower limit near the terrestrial atmosphere
value ($\sim$272). We have also derived the $^{14}$N/$^{15}$N ratio as a
function of the Galactocentric distance and deduced a linear trend based on
unprecedented statistics. The Galactocentric dependences that we have found are
consistent, in the slope, with past works but we have found a new local
$^{14}$N/$^{15}$N value of $\sim$400, i.e. closer to the Prosolar Nebula value.
A second analysis was done, and a parabolic Galactocentric trend was found.
Comparison with Galactic chemical evolution models shows that the slope until 8
kpc is consistent with the linear analysis, while the flattening trend above 8
kpc is well reproduced by the parabolic analysis.
