PURPOSE: To verify the regulation of transepithelial resistance (TER) of Sertoli cells by Leydig cells in mouse. MATERIALS AND METHODS: Primary culture of Sertoli cells was established on cell culture plate insert and monolayer culture was subjected to coculture in the Leydig cell culture. Changes in TER was monitored for 48 h using the conductivity meter equipped with two electrodes system. RESULTS: TER gradually increased according to the development of monolayer of Sertoli cells on the cell culture plate insert. Net changes in TER of Sertoli cells culture was significantly higher under the Leydig cells coculture compared to control after 48 h of coculture. CONCLUSIONS: It is the first report about the increase in TER of Sertoli cells by Leydig cells in vitro. Paracrine interaction between Leydig cells and Sertoli cells might be involved in the development of functional blood testis barrier which is made by tight junctions between Sertoli cells in mouse testis.