The aim of the present work was to study the effect of oestrogen receptor inhibitors (ICI 164.384; ICI and cyclofenil; CF) on proliferation, oestradiol synthesis, aromatase expression and telomerase activity (TA) of pig granulosa cells (GC) derived from small (1-2 mm; SF-GC) and large (5-7 mm; LF-GC) follicles. Cells were treated with anioestrogens for 48 h in basic (medium only) as well as FSH-stimulated conditions. Antioestrogens applied individually and in a combination significantly (P<0.01) decreased proliferative potential of LF-GC. Antioestrogens applied in a combination increased (P<0.05-0.01) 17-β oestradiol synthesis in small and large follicle GC. Antioestrogens applied individually and in a combination caused an increase (P<0.01) of aromatase gene expression in FSH-stimulated conditions as well as in basic conditions. Moreover, antioestrogens increased (P<0.05-0.01) TA in basic and in FSH-stimulated conditions. The results of the study indicate the involvement of oestrogen receptor α and β in the control of proliferation, differentiation and telomerase activity in pig GC. Furthermore, telomerase activity does not have to be linked only to pig GC proliferation but may also be involved in the differentiation process.