We have carried out a long term project to analyse the role played by bars in bulge formation and the formation and evolution of bars from the point of view of their star formation histories and stellar population. To this aim, we have obtained high S/N long-slit spectra of 20 barred galaxies, ranging in morphological types between S0 and Sb galaxies. We have derived Lick/IDS indices along the bar for all the sample galaxies. We find three types of bars according to their metallicity gradient; bars with negative metallicity gradient, bars with null metallicity gradient and galaxies with positive metallicity gradient. Regarding the bulge region we find that the ages and metallicities are younger and more metal rich, at a given σ, than the bulges of unbarred galaxies. The E/Fe derived for the bulges of barred galaxies lie above the values of the unbarred galaxies at a given σ. The gradients of the metal indices in the bulge region tend to be negative, less metal rich towards the end of the bulge. The gradient values show a large scatter for galaxies with σ below 150 kms-1. The Balmer indices show the opposite trend with the scatter of the gradients also increasing for galaxies with σ below 150 kms-1. The bulges of the barred galaxies in our sample have suffered a different chemical enrichment history compared to unbarred galaxies of the same morphological types, same σ distribution and same inclinations.