  Direct imagery and long-slit, spatially resolved echellograms of the high
excitation planetary nebula NGC 1501 allowed us to study in detail the
expansion velocity field, the physical conditions (electron temperature,
electron density, ionization) and the spatial distribution of the nebular gas.
An electron temperature of 11500 K and a turbulence of 18 km/s are derived by
comparing the Halpha and [OIII] emission line profiles, but large, small scale
fluctuations of both these quantities are present in the ionized gas. The
radial density distribution shows external peaks up to 1400 cm-3; they have
steep outwards profiles and extended inwards tails probably originated by
Rayleigh-Taylor instability and winds interaction. The complexity of the
expanding motions indicates that the main part of NGC 1501 is a thin ellipsoid
of moderate ellipticity, but the presence of a pair of large lobes along both
the major and the intermediate axes and of a multitude of smaller bumps spread
on the whole nebular surface, makes the general 3-D structure of NGC 1501 like
a boiling, tetra-lobed shell. This peculiar morphology can be qualitatively
explained in terms of interaction of the slow nebular material with the intense
and fast wind from the WC4/OVI central star.
