The in vitro permeation of thyrotropin-releasing hormone (TRH) through rabbit nasal, rectal and duodenal mucosae was studied in the absence and presence of an enzyme inhibitor and permeation enhancer. TRH in the donor and receptor solutions was assayed by HPLC. When thimerosal (TM, 0.5 mM) was added to the donor cell as an inhibitor, the permeation rate of TRH (200 $mu extrm{g}$/ml) increased linearly as a function of time. Fluxes of TRH through the nasal, rectal and duodenal mucosae were found to be 33.3$pm$5.9, 11.8$pm$1.9 and 9.6$pm$0.7 $mu extrm{g}$/$ extrm{Cm}^2$/hr, respectively. The addition of sodium glycocholate, glycyrrhizic acid ammonium salt, sodium taurodihydrofusidate or L-$alpha$-lysophosphatidylcholine to the donor solution containing TM did not result in the significant increase of permeation flux except for the duodenal mucosa, comparing with that in the presence of TM alone. Consequently, it was suggested that the nasal route was advantageous for systemic delivery of TRH, and the addition of TM and/or an enhancer was necessary to maximize the transmucosal permeation of TRH.