Uterine leiomyoma is the most common tumour in women with a reported incidence of 25-30%. The tumors are benign, composed of smooth muscle cells with variable amount of collagen - rich fibrous tissue. It is well established that accumulation of extracellular matrix in leiomyoma is key feature of tissue fibrosis. However, the pathogenesis of leiomyoma is still unclear. The aim of this study was to evaluate the metabolism of collagen in cultured leiomyoma cells and in control myometrium cells. The effect of estradiol, selective modulators of estrogen receptors (raloxifene, tamoxifen) and estrogen receptor down regulator (ICI 182.780) on collagen biosynthesis (measured by 5-[3H]-proline incorporation assay and measurement of prolidase activity) and collagen degradation (measured by metalloproteinase activity assay) was studied. It was found that collagen biosynthesis is strongly stimulated by low doses of estradiol (5 nM) in leiomyoma cells while it is not changed in control myometrium cells. An increase in estradiol concentration to 10 nM results in drastic decrease of this process both in leiomyoma as well as control cells. Although raloxifene and tamoxifen only slightly affected collagen biosynthesis in control myometrium cells, they significantly inhibited the process in leiomyoma cells. There was no coordinate correlation between collagen biosysignificantly inhibited the process in leiomyoma cells. There was no coordinate correlation between collagen biosynthesis and prolidase activity suggesting that regulation of this process may take place at transcriptional level. Both estrogen and SERMs were found to inhibit MMP-2 in leiomyoma as well as in control myometrium cells. The data suggest that stimulatory action of estrogen on collagen biosynthesis and inhibitory effect on MMP-2 activity in uterine leiomyoma may contribute to accumulation of this protein in ECM of this tissue.