  Observational constraints provided by high resolution and high sensitivity
observations of external galaxies made in the millimeter and submillimeter
range have started to put on a firm ground the study of extragalactic chemistry
of molecular gas. In particular, the availability of multi-species and
multi-line surveys of nearby galaxies is central to the interpretation of
existent and forthcoming millimeter observations of the high redshift universe.
Probing the physical and chemical status of molecular gas in starbursts and
active galaxies (AGN) requires the use of specific tracers of the relevant
energetic phenomena that are known to be at play in these galaxies: large-scale
shocks, strong UV fields, cosmic rays and X-rays. We present below the first
results of an ongoing survey, allying the IRAM 30m telescope with the Plateau
de Bure interferometer(PdBI), devoted to study the chemistry of molecular gas
in a sample of starbursts and AGN of the local universe. These observations
highlight the existence of a strong chemical differentiation in the molecular
disks of starbursts and AGN.
