a b s t r a c t Leptin is a modulator of oocyte maturation and follicular development in swine. The MAPK are serine/threonine kinases that act as signal transduction pathways in swine ovaries. This study evaluated the presence of leptin, activated MAPK ERK 1/2 and p38 in oocytes of primordial/primary, secondary and tertiary follicles of gilts and sows. Ovaries from ten gilts and ten sows were collected in an abattoir, fixed in 10% formalin and prepared with classical histology methods. For immunohistochemistry, slides were incubated with poly-clonal antibodies anti-leptin, anti-phospho ERK1/2 MAPK and anti-phospho p38 MAPK. Leptin immuno-labeling and the presence of activated ERK 1/2 MAPK were more intense for oocytes of sows (P < 0.05), whereas p38 MAPK was more active for oocytes of gilts (P < 0.05). Although no differences in immunolabeling for leptin and p38 MAPK were observed for oocytes of gilts at distinct follicle developmental stages (P > 0.05), immunolabeling was intense for oocytes of sows included in primordial/primary follicles (P < 0.05). Thus, leptin and p38 MAPK may be important to start oocyte development.