Several treatments for patients with primary hypogonadism are available, but these are associated with major complications. In this study, we explored the possibility of testosterone secretion by proliferated Leydig cells embedded in Matrigel with the aim of developing a source of endogenous testosterone supplement for recipients while reducing the need for donor material. Leydig cells were isolated and proliferated in vitro. The expression of 3β-hydroxysteroid dehydrogenase, cholesterol side-chain cleaving enzyme (CYP11A1), and 17α-hydroxylase/17,20-lyase (CYP17A1) was analyzed to confirm the purity and steroidogenesis capability of Leydig cells. The proliferated cells were then embedded in three-dimensional Matrigel, and following culture the supernatant medium was collected for measurement of testosterone concentration by radioimmunoassay. The biological behavior of the Leydig cells in the Matrigel was carefully observed under the microscope. Approximately 6.0 × 10(5) Leydig cells were obtained from one testis after primary culture in vitro. Aliquots of 1.0 × 10(5) Leydig cells were mixed with Matrigel, with the amount of cells in one pellet being equal to that in an adult testis. Leydig cells gradually formed aggregates when maintained in Matrigel. A rapid and constant linear increase in testosterone levels was detected in the supernatant medium. Our results demonstrate that Matrigel is a perfect support matrix for Leydig cells. Proliferated Leydig cells embedded in Matrigel have a great steroidogenesis reserve. In our study, they contributed to continuous steroidogenesis, which implies that the pellet may provide the physiological demand for endogenous androgen once engrafted in vivo. This system may ultimately provide a novel alternative treatment for people who are in need of androgen replacement.