We have demonstrated earlier that thyroid hormone (T-3) regulates the activity of cholinephosphotransferase (CPT) in guinea pig lung. This effect of T-3 is not organ specific because we found T-3 also regulates CPT activity in the guinea pig liver. Northern blot analysis using two oligonucleotide probes, one synthesized on the basis of the yeast CPT gene sequence and another on the basis of partial cDNA clone from guinea pig CPT clone, revealed that T-3 stimulates the expression of new CPT mRNA, Studies with transcriptional and translational inhibitors indicated that T-3 enhanced the translation of the CPT mRNA as well as translocation of preformed CPT enzyme protein from cytosol to mitochondria, Furthermore, it strengthens our previous finding that yeast CPT and guinea pig CPT have high homology in their sequence as both the oligonucleotide probes gave the similar type of Northern blot in the present study.