Temporins are a group of small, highly hydrophobic, antimicrobial peptides widely distributed in the skin of frogs from the Ranidae family. In order to examine the mechanisms of regulation of temporin gene expression, we measured expression levels of preprotemporin mRNA in the skin of the Japanese mountain brown frog Rana ornativentris, using a semiquantitative RT-PCR system. Preprotemporin mRNAs were not detected in skin prior to the onset of metamorphosis but their levels increased markedly during metamorphosis, reaching a maximum at the stages of metamorphic climax, suggesting a correlation with thyroid hormone concentrations. Consequently, we examined direct effects of triiodothyronine (T3) on in vivo preprotemporin gene expression. Treatment of adult animals with 2 × 10-9 mol/L T3 for 48 h raised the preprotemporin mRNA levels in skin by 1.5-fold compared with untreated controls.