It is well established that an increased availability of tryptophan stimulates serotonin synthesis not only in the adult but also in the developing brain. In order to study the influence of a permanently increased supply of tryptophan on the developing central 5-HT-system, female rats were fed a tryptophan-enriched diet from mating throughout pregnancy and lactation. The effect of this dietary regime was assessed by measurements of neurochemical markers of 5-HT innervation in the developing brain of their offspring. A diminished content of 5-HT, a decreased activity of tryptophan hydroxylase and a reduction of crude synaptosomal high-affinity 5-HT uptake was found in the cortex and in the brain stem of 5 day old rat pups of mothers fed the tryptophan-enriched diet. The postnatal increase of all three markers of serotonergic innervation in the offspring of these mothers was retarded. Both the initial depletion and the delayed maturation of 5-HT content, of tryptophan hydroxylase activity and of synaptosomal serotonin uptake were more pronounced in the cortex than in the brain stem. Apparently, the increased dietary intake of tryptophan throughout pregnancy and lactation caused a delayed outgrowth of 5-HT axons and/or reduced collateral sprouting and synapse formation in the brain of the developing offspring.