Carboxypeptidase-H (CPH) is a metallocarboxypeptidase implicated in the processing of peptide hormones. Consistent with such a role, the gene for CPH is expressed in cells that secrete regulatory peptides, such as those of the brain and endocrine tissues. In the rat brain, CPH is transcribed from a single transcriptional start site associated with the initiator-type element first described in the gene encoding lymphocyte-specific terminal deoxynucleotidyltransferase. We have used a combination of Northern blot analysis and S1 nuclease protection mapping to describe the expression and transcription initiation pattern of the gene for CPH in the peripheral tissues and brain regions of the normal rat. The single transcriptional start site is used in all tissues examined, except the pituitary, where two additional specific initiation sites are found. The expression of the CPH gene is up-regulated in the anterior pituitary gland as a consequence of systemic thyroid hormone depletion, and this increase is associated with a preferential utilization of the novel upstream transcriptional initiation sites. Thus, the use of different major transcriptional initiation sites of the CPH gene in the pituitary gland is subject to differential direct or indirect thyroid hormone regulation.