  We show that the excess of nitrogen emission observed in a large sample of
Starburst Nucleus Galaxies (SBNGs) can only be explained at a given metallicity
by an overabundance of nitrogen with respect to normal HII regions in the disks
of late-type spirals. The N/O ratios in the SBNGs are comparable to the values
found in the bulges of normal early-type spirals, which suggests that what we
observe could be the main production of nitrogen in the bulges of these
galaxies.
  The variation of the N/O ratio as a function of metallicity in SBNGs follows
a primary + secondary relation, but the increase of nitrogen does not appear as
a continuous process. In SBNGs, nitrogen is probably produced by different
populations of intermediate-mass stars, which were formed during past sequences
of bursts of star formation. This assumption pushes the origin of the main
bursts 2-3 Gyrs back in the past. On a cosmological scale, this time interval
corresponds to redshifts z = 0.2-0.3, where a significant increase of star
formation activity occurred. The origin of the SBNG phenomenon would thus have
cosmological implications, it would be related to a more active phase of star
formation in the Universe sometime in its recent past.
