We present an experiment in which lips are perturbed by a 20-mm diameter tube, for the production of the French vowel [u]. Nine naive speakers are analysed in 3 successive sessions: (1) without perturbation; (2) with the lip-tube; (3) with the lip-tube and with some information to help them to compensate. The analysis is based on X-ray views and formant patterns. Compensation abilities are evaluated by considering the first two formants. After the 2nd session, only one speaker could compensate the perturbation. All other speakers showed a strong increase of F2. In the 3rd session, behaviours of the speakers were observed depending on the given information: articulatory, acoustic or both. Results are analysed in the light of the classical control theories of speech production.