  I present deep spectroscopy of four HII regions in the inner, metal-rich zone
of the spiral galaxy M101 obtained with the LRIS spectrograph at the Keck
telescope. From the analysis of the collisionally excited lines in two of the
target HII regions, H1013 and H493, I have obtained oxygen abundances
12+log(O/H)=8.52 and 12+log(O/H)=8.74, respectively. These measurements extend
the determination of the oxygen abundance gradient of M101 via the direct
method to only 3 kpc from the center. The intensity of the CII 4267 line in
H1013 leads to a carbon abundance 12+log(C/H)=8.66, corresponding to nearly
twice the solar value. From a comparison of the continuum temperature derived
from the Balmer discontinuity, T(Bac)=5000 K, and the line temperature derived
from [OIII]4363/5007, T[OIII]=7700 K, an average temperature T0=5500 K and a
mean square temperature fluctuation t^2=0.06 have been derived. Accounting for
the spatial inhomogeneity in temperature raises the oxygen abundance obtained
from the oxygen auroral lines to 12+log(O/H)=8.93. These findings are discussed
in the context of the calibration of strong-line metallicity indicators, in
particular of the upper branch of R23. There is no evidence for the strong
abundance biases arising from temperature gradients predicted theoretically for
metal-rich HII regions.
