  We present long-slit observations in the optical and near infrared of eight
HII regions in the spiral galaxy NGC 4258. Six of the observed regions are
located in the SE inner spiral arms and the other two are isolated in the
northern outer arms. A detailed analysis of the physical conditions of the gas
has been performed. For two of the regions, an electron temperature has been
derived from the [SIII] 6312 A line. For the rest, an empirical calibration
based on the red and near infrared sulphur lines has been used. The oxygen
abundances derived by both methods are found to be significantly lower (by a
factor of two) than previously derived by using empirical calibrations based on
the optical oxygen lines. In the brightest region, 74C, the observation of a
prominent feature due to Wolf-Rayet stars provides an excellent constraint over
some properties of the ionizing clusters. In the light of the current
evolutionary synthesis models, no consistent solution is found to explain at
the same time both the WR feature characteristics and the emission line
spectrum of this region. In principle, the presence of WR stars could lead to
large temperature fluctuations and also to a hardening of the ionizing
radiation. None of these effects are found in region 74C for which the electron
temperatures found from the [SIII] 6312A and the Paschen discontinuity at 8200A
are equal within the errors and the effective temperature of the ionizing
radiation is estimated at around 35,300K. Both more observations of confirmed
high metallicity regions and a finer metallicity grid for the evolutionary
synthesis models are needed in order to understand the ionizing populations of
HII regions.
