It has been hypothesised that rat lines genetically selected for their alcohol preference consume large amounts of ethanol because they have a low 5-HT content. Since brain tryptophan (TRP) availability controls the rate at which neurons synthesise and release serotonin (5-HT), we assessed whether the administration of a TRP-supplemented or TRP-free diet for 3 consecutive days influenced alcohol intake in alcohol-preferring and non-preferring sP and sNP rats, respectively. In the same animals extracellular 5-HT concentration was monitored by microdialysis in the frontal cortex. A TRP-free diet progressively and markedly decreased cortical extracellular 5-HT in sP and sNP rats during the treatment period with respect to a balanced diet. However, the TRP-free diet failed to modify alcohol consumption and preference in sP and sNP rats. The TRP-supplemented diet also failed to alter the intake of alcohol in either group of rats. Therefore, these results do not support a specific role of 5-HT transmission in ethanol intake and preference in sP and sNP rats.