  We present mid-infrared ISO-SWS observations of the fine-structure emissions
lines [NeII]12.8, [NeIII]15.6, [NeIII]36, [ArII]6.9, [ArIII]8.9, [SIII]18.7,
[SIII]33.5, [SIV]10.5, and the recombination lines Br_alpha and Br_beta, in a
sample of 112 Galactic HII regions and 37 nearby extra-galactic HII regions in
the LMC, SMC, and M33. The Galactic sources have a wide range in galactocentric
distance (0<R<18kpc), which enables us to study excitation and metallicity
variations over large Galactic scales. We detect a steep rise in the
[NeIII]15.6/[NeII]12.8,[ArIII]8.9/[ArII]6.9, and [SIV]10.5/[SIII]33.5
excitation ratios from the inner Galaxy outwards, and a moderate decrease in
metallicity, from twice solar in the inner Galaxy to solar in the outer disk.
The extra galactic sources in our sample show low gas density, low metallicity
and high excitation. We find a good correlation between the neon and the argon
excitation ratios in our sample. The observed correlation is well-reproduced by
theoretical nebular models which incorporate new generation wind-driven NLTE
model stellar atmospheres for the photoionizing stars. In particular, the NLTE
atmospheres can account for the production of [NeIII] emission in the HII
regions. We conclude that the increase in nebular excitation with
galactocentric radius is due to an increase in stellar effective temperature
(as opposed to a hardening of the stellar SEDs due to the metallicity
gradient). The good fit between observations and our models support the
conclusion of Thornley et al. (2000), that the low neon ratios observed in
extra-galactic sources are due to global aging effects.
