To document the subcellular compartmentalization of the tyrosine hydroxylase (TH) primary (heterogeneous nuclear, hnRNA) and processed mRNA transcripts in vivo, we have studied the expression of the TH gene in the locus coeruleus of the rat brain in basal conditions and under pharmacological stimulation by reserpine. Using exon-specific probes and electron microscopic in situ hybridization in reserpine-treated animals, we have demonstrated that the TH mRNA is localized both in the perikaryal cytoplasm (in association with domains of the endoplasmic reticulum) and in the nucleus (in large and smaller aggregates). By comparing hybridization patterns with intron- and exon-specific probes, we have shown that the two main foci of nuclear TH RNA labeling correspond to the primary (hnRNA) transcript, while accessory secondary tracks or dots represent mature transcript devoid of introns. Finally, our data indicate that the pattern of expression of the TH gene is heterogeneous within the locus coeruleus neuron population. Under basal conditions, many neurons exhibit no detectable intron signal, although all locus coeruleus neurones express the TH gene. Other neurons display high intron labeling, and obviously express the gene at a much higher level. Upon reserpine stimulation, the number of neurons displaying detectable intron signals, as well as the intensity of the intron signal per neuron progressively increases during the first 24 h, suggesting that TH expressing neurons are progressively recruited for higher expression of this gene as the stimulation progresses.