The main component of protoplanetary nebulae (PPNe) is thought
 to be neutral or slightly ionized atomic gas. In fact, such a
 composition is characteristic of PPNe, evolutionarily placed
 between the (commonly) molecule-rich AGB envelopes and the
 strongly ionized planetary nebulae. Since the PPNe still
 present a low excitation, the observation of such atomic
 component should be done by means of the fine-structure lines.
 For well known technical reasons, however, it is difficult
 today to detect such lines, and the observation of PPNe has
 been carried out quite indirectly. The fact that we are
 missing the main observational information suggests that our
 ideas on the conditions and evolution during the PPN phase may
 be severely incomplete. A drawback that ISO observations could
 obviously overcome.

 We propose, in summary, to observe a number of fine-structure
 atomic lines in a sample of PPNe. The line list includes
 transitions of different atoms and with different excitation
 requirements, in order to provide direct information on the
 total mass, composition and excitation state of PPNe. We also
 intend to analyse the signature of the different winds in the
 spectra, by means of observations with high spectral
 resolution. The source sample is wide enough to represent the
 various kinds of PPNe, i.e. objects with different evolution
 degree or with different initial mass and composition.