  In star forming regions in our own Galaxy, the 14N/15N ratio is found to vary
from $\sim$ 100 in meteorites, comets and protoplanetary disks up to $\sim$
1000 in pre-stellar and star forming cores, while in external galaxies the very
few single-dish large scale measurements of this ratio lead to values of
100-450. The extent of the contribution of isotopic fractionation to these
variations is, to date, unknown. In this paper we present a theoretical
chemical study of nitrogen fractionation in external galaxies in order to
determine the physical conditions that may lead to a spread of the 14N/15N
ratio from the solar value of $\sim$440 and hence evaluate the contribution of
chemical reactions in the ISM to nitrogen fractionation. We find that the main
cause of ISM enrichment of nitrogen fractionation is high gas densities, aided
by high fluxes of cosmic rays.
