The present study provides the evidence that pig granulosa cells originating from small (SF) and large (LF) ovarian follicles differ in proliferative and differentiative capacities in vitro. SF showed significantly higher proliferation potential in vitro, LF showed significantly advanced steroidogenic functions. Any substantial difference in average telomerase activity between freshly isolated SF and LF was not detected. When cultured, both SF and LF were responsive to EGF and FSH treatment and expressed absolutely higher levels of telomerase activity in comparison to freshly isolated cells. Except of basic and EGF conditions at a 24 h interval, no significantly different levels of telomerase activity between the SF and LF at an interval of 24 and 48 hof in vitro culture were observed. The activity of telomerase in pig ovarian follicle development may not be linked only to granulosa cell proliferation but some regulatory mechanisms of differentiation might be involved, too.