Abstract: This study was done to examine immunohistochemically the presence of LIF, LIFR and gp130 in the monkey ovary during the menstrual cycle. The results showed that the immunostaining for LIF, LIFR and gp130 was cycle-dependent, was considerably higher during proliferative phases than during secretory phases of the menstrual cycle. The oocytes at all stages of development were the primary site of immunoreactive LIF, LIFR and gp130, which were present to a lesser extent in the granulose cells, theca cells, stromal cells and serosal tissue. No immunostaining for LIF, LIFR and gp130 was found in atretic follicles, regressed corpora lutea. The cycle dependency suggests a potential regulation of their expression by ovarian steroids. LIF may play an important role in the regulation of monkey follicular development and ovulation.