New controlled-release formulations containing tryptophan and/or its metabolites are described, as well as the process to obtain them in tablet forms suitable for oral administration in the treatment of pathologies or conditions related to serotonin deficiency in the Nervous System. The formulations are comprised in double-layer tablets, one layer containing 5-hyrohytryptophan released rapidly (phase “fast), the other layer containing tryptophan or 5-hydroxytryptophan, progressively released (“retarded”). The finished product is characterized by optimal differential release profile under physiological conditions. In all cases the release kinetics obtained is accurately time- and concentration-controlled, therefore avoiding fortuitous release of the active ingredients, and in clinical setting is free of undesirable side effects.