Leptin is an important satiety hormone and reproductive regulator found, along with its receptors, throughout the ovary. To date, the changes in ovarian expression of leptin receptor during the prepubertal period and throughout the estrous cycle in the pig have not been studied. In this study, the long form of the leptin receptor was a detectable level in immature pig ovarian follicles when assayed using semiquantitative reverse transcription-polimerase chain reaction (RT-PCR). Moreover, its level was increased in growing follicles during follicular phase of the estrous cycle (6-fold in early antral [EAF] and antral [AF] follicles) and was highest in newly formed corpora lutea. Its changes paralleled those in steroid secretion and especially progesterone (P₄) secretion. Additionally, we showed that insulin-like growth factor (IGF)-I had stimulatory effect on leptin receptor expression using a tissue culture model of follicular and luteal cells, with a 12-fold increase in prepubertal ovaries and a 1.5- to 2-fold increase in growing follicles and newly formed corpora lutea (CL). These results suggest that ovarian expression of leptin receptor is regulated throughout the estrous cycle by ovarian steroids, with peak expression at ovulation, indicating a possible involvement in follicular development and corpus luteum formation. Moreover, this data points to an important role of IGF-I in leptin receptor expression during the entire estrous cycle, with a special role during the prepubertal period.