Nearby galaxies are ideal places to study in detail metallicity gradients and their time evolution. We consider chemical abundances of a new sample of hii regions complemented with previous literature data-sets. We compare hii region and PN abundances obtained with a common set of observations taken at MMT. With an updated theoretical model, we follow 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. Supported by a uniform sample of nebular spectroscopic observations, we conclude that: {em i}) the metallicity distribution in M33 is very complex, showing a central depression in metallicity probably due to observational bias; {em 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; {em iii}) there is a small evolution of the slope with time from the epoch of PN progenitor formation to the present-time.}
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