Abstract: Objective To explore the relationship between Leydig cell tumor and levels of sex hormones and the expression levels of AR，LH and ER in Leydig cells in male Fisher344 rats.Methods Male Fisher344，Sprague D awl eg，Wistar rats were respectively divided into 4-month-old group（group A₁，B₁，C₁），7-mo nth-old group（group A₂，B₂，C₂），10-month-old group（group A₃，B₃，C₃），13-month-old group（group A₄，B₄，C₄）.Each group comprised of ten rats.In addition，another group was five 26-month-old male Fisher344 rats（group L）.Pathologic changes in testes were observed with microscope.Testes of group L were observed with transmission electron microscope.Immuno-histochemistry analysis of androgen receptor，luteinizing hormone，estrogen receptor were taken in tests by S ABC method，in every group except group L.The levels of testosterone（T），luteinizing hormone（LH），estradiol（E2），prolactin（PRL），follicle-stimulating hormone（FSH）and progesterone（P）in sera of each group were measured with radioimmunological analysis method.Results Hyperplasia of Leydig cell occurred in group A₃，and more hyperplasia in group A₄，and focal hyperplasia had formatted in some cases.All testes of group L had generated Leydig cell tumor.The expression level of AR in Leydig cells in group A₄ was higher than that of group C₃，B₄，C₄（P＜0.05）.The expression levels of LH in Leydig cell in group A₃ and A₄ were more than that of group B₃，B₄ and C₄（P＜0.05）.The expression level of ER in Leydig cell in 1 group A₄ was less than that of group Bi，B₄ and C₄（P＜0.05）.The level of sera T of group A₃ or A₄ was higher than that of group B₃，B₄，C₃ or C₄（P＜0.01）.Group L has the lowest level of sera T and it was lower than that of the other groups（P＜0.05 or P＜0.01）except group C₄（p≥0.05）.There were no significant differences between level of sera LH group L and that of group A₄，B₄，C₄（≥0.05）.The level of sera E2 of group L was the highest，which was significantly higher than that of the other groups（P＜0.05）except group Bi and B₂（p≥0.05）.The level of sera PRL of group L was significantly higher than that of group A₄（P＜0.05），and one of both groups was respectively higher than other groups（P＜0.01）.There is no significant difference of the level of sera FSH of the same age groups（P≥0.05），and the levels of sera FSH cf group L，A₄，B₄ and C₄ were almost the same.The level of sera P of group L was significantly higher than that of the other groups（P＜0.01）.Conclusions Most of the Leydig cell tumors in male Fisher344 rats were benign tumor.Leydig cell tumor in male Fisher344 rats may be developed from hyperplasia and focal hyperplasia of Leydig cell.There was a significant correlation between the relatively more expression level of LH and AR in Leydig cell and hyperplasia and focal hyperplasia of Leydig cell in male Fisher344 rats.The development of spontaneous Leydig cell tumors in old male Fisher344 rats may be related to their extremely low level of sera T and the relatively low level sera E2 and P but uncorrelated with the level of sera FSH and LH.The relatively high level of sera PRL may be related to Leydig cell hyperplasia and tumor generation.

Key words: Fisher344 rats, Spontaneous Leydig cells tumor, Immuno-histochemistry, Serum Hormone