Context : Nearby galaxies are ideal places to study metallicity gradients in detail and their time evolution .

Aims : We analyse the spatial distribution of metals in M33 using a new sample and the literature data on H ii regions , and constrain a model of galactic chemical evolution with H ii region and planetary nebula ( PN ) abundances .

Methods : We consider chemical abundances of a new sample of H ii regions complemented with previous data sets .
We compared H ii region and PN abundances obtained with a common set of observations taken at MMT .
With an updated theoretical model , we followed the time evolution of the baryonic components and chemical abundances in the disk of M33 , assuming that the galaxy is accreting gas from an external reservoir .

Results : From the sample of H ii regions , we find that i ) the 2D metallicity distribution has an off-centre peak located in the southern arm ; ii ) the oxygen abundance gradients in the northern and southern sectors , as well as in the nearest and farthest sides , are identical within the uncertainties , with slopes around -0.03-4 dex kpc ^ { -1 } ; iii ) bright giant H ii regions have a steeper abundance gradient than the other H ii regions ; iv ) H ii regions and PNe have O/H gradients very close within the errors ; v ) our updated evolutionary model is able to reproduce the new observational constraints , as well as the metallicity gradient and its evolution .

Conclusions : Supported by a uniform sample of nebular spectroscopic observations , we conclude that i ) the metallicity distribution in M33 is very complex , showing a central depression in metallicity probably due to observational bias ; ii ) the metallicity gradient in the disk of M33 has a slope of -0.037 \pm 0.009 dex kpc ^ { -1 } in the whole radial range up to \sim 8 kpc , and -0.044 \pm 0.009 dex kpc ^ { -1 } excluding the central kpc ; iii ) there is little evolution in the slope with time from the epoch of PN progenitor formation to the present .