The invention concerns a process for preparing a tablet comprising at least two layers, wherein one layer is "fast" release and comprises 5-hydroxytryptophan and the other layer is "retard" release and comprises 5-hydroxytryptophan or tryptophan. The tablet obtained according to the process of the invention, at 24 months of storage, exhibits a release of more than 90% of 5-HTP from the "fast" layer within 5 minutes of administration, of about 50-70% of Trp or 5-HTP from the "retard" layer within 4 hours and of more than 80% of Trp or 5-HTP from the "retard" layer within 8 hours.
