We present our observations of several planetary nebulae in the central zones of the spiral galaxy M33. We combine the abundances we derived for our objects with those obtained by Magrini et al. (2004) for a sample of PNe at larger galactocentric distances. We compare the radial abundance distribution derived from planetary nebulae with the one derived by HII regions and massive stars. We discuss the uncertainties and biases of the abundance derivations in planetary nebulae and HII regions. We show that planetary nebulae, although difficult to observe, are an important tool to investigate the chemical evolution of galaxies.