Conclusions. We find observational evidence that the presence of a galactic bar affects the properties of the ionised gas in the central parts of disc galaxies (radii. less than or similar to 0.6-2.1 kpc). The most striking effect is an enhancement in the N/O abundance ratio. This can be interpreted qualitatively in terms of our current knowledge of bar formation and evolution, and of chemical evolution models, as being the result of a different star formation history in the centres of barred galaxies caused by the gas inflow induced by the bar. Our results lend support to the scenario in which less massive and more massive bulges have different origins or evolutionary processes, with the gaseous phase of the former currently having a closer relation to the bars.