Insulin-like growth factor 1 (IGF1) is involved in the proliferation of mouse and rat endometrial cells in a paracrine or autocrine manner. Insulin-like growth factor binding protein-3 (IGFBP3) modulates actions of IGFs directly or indirectly. The present study aimed to determine whether IGFBP3 is involved in the regulation of proliferation of mouse endometrial cells. Mouse endometrial epithelial cells and stromal cells were isolated, and cultured in a serum free medium. IGF1 stimulated DNA synthesis by endometrial epithelial and stromal cells, and IGFBP3 inhibited IGF1-induced DNA synthesis. Estradiol-17β (E2) decreased the Igfbp3 mRNA level in endometrial stromal cells, whereas it increased the Igf1 mRNA level. Transforming growth factor α (TGFα) significantly decreased IGFBP3 expression at both the mRNA and secreted protein levels in endometrial stromal cells. Progesterone (P4) did not affect the E2-induced down-regulation of Igfbp3 mRNA expression in endometrial stromal cells, although P4 alone increased Igfbp3 mRNA levels. The present findings suggest that in mouse endometrial stromal cells E2 enhances IGF1 action through enhancement of IGF1 synthesis and reduction of IGFBP3 synthesis, and that TGFα affects IGF1 actions through modulation of IGFBP3 levels.