Introduction. In the last few years, various studies have underlined a correlation between thyroid function and male sexual function, hypothesizing a direct action of thyroid hormones on the penis. Aim. To study the spatiotemporal distribution of mRNA for the thyroid hormone nuclear receptors (TR) alpha 1, alpha 2 and beta in the penis and smooth muscle cells (SMCs) of the corpora cavernosa of rats and humans during development. Methods. We used several molecular biology techniques to study the TR expression in whole tissues or primary cultures from human and rodent penile tissues of different ages. Main Outcome Measure. We measured our data by semi-quantitative reverse transcription polymerase chain reaction (RT-PCR) amplification, Northern blot and immunohistochemistry. Results. We found that TR alpha 1 and TR alpha 2 are both expressed in the penis and in SMCs during ontogenesis without development-dependent changes. However, in the rodent model, TR beta shows an increase from 3 to 6 days post natum (dpn) to 20 dpn, remaining high in adulthood. The same expression profile was observed in humans. While the expression of TR beta is strictly regulated by development, TR alpha 1 is the principal isoform present in corpora cavernosa, suggesting its importance in SMC function. These results have been confirmed by immunohistochemistry localization in SMCs and endothelial cells of the corpora cavernosa. Conclusions. The presence of TRs in the penis provides the biological basis for the direct action of thyroid hormones on this organ. Given this evidence, physicians would be advised to investigate sexual function in men with thyroid disorders. Carosa E, Di Sante S, Rossi S, Castri A, D'Adamo F, Gravina GL, Ronchi P, Kostrouch Z, Dolci S, Lenzi A, and Jannini EA. Ontogenetic profile of the expression of thyroid hormone receptors in rat and human corpora cavernosa of the penis. J Sex Med 2010;7:1381-1390.