Abstract: The effects of a single dose of p-chlorophenylalanine on the mRNA encoding tryptophan-5-hydroxylase (EC 126.96.36.199) in the rat dorsal raphe nucleus were analyzed using in situ hybridization. The levels of tryptophan-5-hydroxylase mRNA were markedly increased in cell bodies located in the ventromedial part of the dorsal raphe 1–2 days after p-chlorophenylalanine (300 mg/kg, i.p.) administration. This was followed by a decrease in the amount of tryptophan-5-hydroxylase mRNA, which returned to basal values by 5 days after treatment. An almost symmetric time course was observed for the midbrain serotonin concentration. Our results on the temporal pattern of changes in tryptophan-5-hydroxylase mRNA levels in the ventromedial part of the dorsal raphe are opposite to those reported for the enzyme activity and serotonin concentration after p-chlorophenylalanine treatment. These changes may result from modifications in enzyme mRNA expression, suggesting that tryptophan-5-hydroxylase gene transcription is involved in feedback mechanisms regulating serotonin synthesis.