Water and carbon dioxide have been detected in the stratospheres of the outer planets. Because both species condense at the tropopause (except CO2 in Jupiter and Saturn), their presence in the stratospheres implies an external origin (interplanetary dust, sputtering from the satellites and/or rings, large meteoritic impacts). Carbon monoxide has been detected in each giant planet. Because this compound does not condense at the tropopause, it can also have an internal origin. The source of CO has been proved to be internal and external in Jupiter and Neptune, but this is still not clear in the case of Saturn and Uranus. Therefore, constraining the amount of CO in the stratosphere and/or high troposphere of these planets would help solve this question. We performed observations of Saturn at millimeter and submillimeter wavelengths in the CO (1-0), (2-1) and (3-2) lines. Observations were carried out with the IRAM 30-m telescope (Pico Veleta, Spain) in September 2006 and with the JCMT 15-m telescope (Hawaii, USA) in January 2008. We have recorded broad multi-band spectra. We have used our photochemical model to constrain the CO supply in the atmosphere of Saturn. The results of these observations will be presented and discussed. IRAM is supported by INSU/CNRS (France), MPG (Germany), IGN (Spain). This work has benefited from research funding from the European Community's Sixth Framework Program under RadioNet contract R113CT 2003 5058187.< Réduire