Observed line ratios L(5007)/L(Hα + [N ii]) for PNe of the bulge of M 31 (Ciardullo et al. 2002) and the disk of M 33 (Magrini et al. 2000), as a function of M(5007), compared with the predictions of various hydrodynamical sequences with different metallicities. Evolution proceeds always from faint magnitudes and low line ratios up to maximum 5007 Å emission and then back again, but at higher line ratios. Foreground extinction is considered for the data, but not internal extinction. The arrow indicates the direction of shifts to be applied to the tracks if additional internal extinction occurs.