  The determination of the heavy element abundances from giant extragalactic H
II regions is based on collisionally excited lines. We argue that in the
presence of temperature variations the abundances determined are lower limits
to the real heavy element abundances. To determine the real abundances it is
necessary to take into account the temperature variations present in these
nebulae. We discuss the relevance of obtaining accurate line intensities of
recombination lines of H, He, C, and O to determine the chemical composition of
extragalactic H II regions. We suggest that Pagel's method to derive the O/H
ratio should be calibrated by using recombination lines instead of
photoionization models or abundances derived from collisionally excited lines.
