We have studied the time variation of the radial abundance gradients using samples of planetary nebulae , open clusters , cepheids and other young objects .
Based on the analysis of O/H and S/H abundances for planetary nebulae and metallicities of the remaining objects , we concluded that the gradients have been flattening out in the last 8 Gyr with an average rate of the order of 0.005 – 0.010 dex kpc ^ { -1 } Gyr ^ { -1 } .
We have estimated the errors involved in the determination of the gradients , and concluded that the existence of systematic abundance variations is more likely than a simple statistical dispersion around a mean value .