  Estimates of the gas temperature in planetary nebulae obtained from the [O
III] emission line ratio and from the Balmer discontinuity indicate differences
reaching up to 6000 K (Liu and Danziger 1993). The [O III] temperature is
commonly used to obtain the ionic fractions of highly ionized ions,
particularly the O++ and Ne++ ions when using the empirical method to calculate
the elemental abundances of photoionized gas from the observed emission line
intensities. However, if the gas temperature is overestimated, the elemental
abundances may be underestimated, in particular, it may lead to an uncorrect
elemental abundance gradient of the Galaxy, usually used as a constraint for
the chemical evolution models. Using Monte Carlo simulations, we calculate the
systematic error, introduced in the abundance gradient obtained from planetary
nebulae by an overestimation of the gas temperature. The results indicate that
the abundance gradient in the Galaxy should be steeper than previously assumed.
