Post Asymptotic Giant Branch (AGB) phase of evolution is one of the most dynamically developing fields in astrophysics after the IRAS mission. Objects in this phase of evolution, called proto-planetary nebulae (PPNe) are formed during evolution along the AGB when mass of the H-rich envelope drops to very small values mainly as a result of the large scale mass loss process. PPNe are very intriguing objects because of rapid changes in the stellar temperature and, in consequence, in the physical conditions inside the ejected shell and the remnant stellar atmosphere. Their transitory nature can be seen in dust properties which are manifested by peculiar bands (e.g. the 21 micron band discovered by Kwok et al., 1989). One of the intriguing aspect of post-AGB evolution is existence of the fast winds detected for some of them. Such winds will interact with the envelope ejected on the AGB producing specific excitation conditions for the molecules present inside the shells. The goal of our proposal is to detect CO far-IR lines and compare their relative strengths in order to analyze excitation conditions inside the envelopes.