Peptidylglycine alpha-amidating monooxygenase (PAM; EC 1.14.17.3) is a multifunctional protein containing two enzymes that act sequentially to catalyze the alpha-amidation of neuroendocrine peptides. The regulatory mechanism(s) involved in the tissue-specific induction of PAM messenger RNA (mRNA) by thyroid status have been investigated in rat anterior pituitary gland. In this tissue, cellular PAM mRNA increases in response to hypothyroidism (4- to 7-fold above basal levels). To gain further insight into this pretranslational control, nuclear in vitro run-on transcription assays were performed. Using PAM complementary DNAs and intronic probe, we showed that the transcriptional rate of rat pituitary PAM gene in isolated nuclei was not altered by thyroid status. Primary rat pituitary cells cultures from hypo- and euthyroid rats in the presence of actinomycin D showed that hypothyroidism increased the half-life of PAM mRNA from 9-10 h to 15-17 h. Taken together, these data suggest that hypothyroidism induces PAM mRNA levels by increasing its stability in the cytoplasm.