  We present ISO-SWS and ISOPHOT-S spectroscopy of the Seyfert galaxy NGC 4151.
We detect a total of 17 fine structure emission lines emitted by a wide range
of low- and high-excitation ions, two rotational lines of molecular hydrogen,
and the Br beta HI line.
  We find that the mid-IR fine structure line profiles display blue asymmetries
which are very similar to those observed in the optical lines produced in the
narrow line region. Because the mid-infrared lines are much less sensitive to
extinction than are the optical lines this similarity places strong constraints
on scenarios which have been invoked to explain the optical line asymmetries.
For example, we are able to rule out the simplest radial-motion-plus-dust
scenarios for the production of the line asymmetries. Our preferred model is
that of a central, geometrically thin but optically thick, obscuring screen of
sub-arcsecond extension, enclosing a total hydrogen gas mass of >~5E6 Msun.
This mass may be molecular.
  The weakness of `PAH' emission features in the low resolution spectrum is
evidence that star formation plays a minor role in the circumnuclear region of
NGC 4151. In a companion paper (Alexander et al., accepted by ApJ), we use the
rich set of mid-infrared lines to determine the obscured photoionizing
continuum produced by the active galactic nucleus.
