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.