Abstract:The pattern of ontogenetic development of tryptophan (TP), tryptamine (T), indole‐3‐acetic acid (IAA), 5‐hydroxytryptamine (5‐HT; serotonin), and 5‐hydroxyindole‐3‐acetic acid (5‐HIAA) in the brains of rats aged 1–45 days is presented. Analysis of the five components in each brain allows the calculation of the acid/amine and amine/amino acid ratios. These metabolic indexes are a useful tool to study and compare the metabolic origins and fates of both amines. The ontogenetic patterns of TP, T, and IAA are very similar, especially during the first week postpartum. The highest and lowest levels found for T were 2.2 ng/g and 0.1 ng/g at the 1st and 5th day, respectively. The temporal relationship between the T/TP and IAA/T ratios suggests the existence of mechanisms protecting T against monoamine oxidase (MAO) which develop in parallel to synaptogenesis. Significant correlations were found between TP and IAA during the whole period studied and between TP and T during the first week after birth. The 5‐HT peak found during the first postpartum week could be due to a non‐neuronal pool of 5‐HT protected against MAO and possibly contained in mast cells. Preliminary determinations on leptomeningeal membranes suggest the existence of such a pool.