  We have obtained multi-object spectroscopy of H II regions in the spiral
galaxy M51 with the Keck I telescope and the Low Resolution Imaging
Spectrometer. For 10 objects we have detected the auroral line [N II] 5755,
while [S III] 6312 has been measured in 7 of these. This has allowed us to
measure the electron temperature of the gas and to derive O, S and N abundances
for the 10 H II regions. Contrary to the expectations from previous
photoionization models of a few H II regions in M51 and from strong-line
abundance indicators, the O/H abundance is below the solar value for most
objects, with the most metal-rich H II regions, P 203 and CCM 72, having
log(O/H) = -3.16 and log(O/H) = -3.29, respectively. The reduction of O/H by
factors up to two or three with respect to previous indirect determinations has
important consequences for the calibration of empirical abundance indicators,
such as R23, in the abundance and excitation range found in the central regions
of spiral galaxies. Based on our new abundances we revise the effective yield
for M51, now found to be almost four times lower than previous estimates, and
we discuss this result within the context of chemical evolution in galactic
disks. Features from Wolf-Rayet stars are detected in a large number of H II
regions in M51, with the C III 5696 line found preferentially in the central,
most metal-rich objects. (abridged)
