The expression of genes encoding opioid precursors: proopiomelanocortin (POMC), proenkephalin (PENK) and prodynorphin (PDYN) was studied in the porcine endometrial explants in the presence or absence of selected cytokines (interleukin-1β (IL-1β), interleukin-6 (IL-6) and tumor necrosis factor α (TNFα)) on days 10–11, 12–13, 15–16 of the oestrous cycle and pregnancy. In pregnant pigs, these days among others are important for 1. migration of the embryos, 2. maternal recognition of pregnancy and 3. the onset of implantation, respectively. During the oestrous cycle, the basal (in the absence of cytokines) mRNA expression of PDYN, but not POMC or PENK was increased on days 15–16 compared to days 10–11. During pregnancy, the gene expression of POMC increased on days 10–11 and 15–16 (vs days 12–13), but PENK – on days 12–13 and 15-16 (vs days 10–11) as well as PDYN – on days 15-16 (vs days 12–13) decreased. The endometrial expression of studied genes was influenced by cytokines. The expression of PENK was stimulated by IL-6 in all studied days of the cycle as well as on days 10–11 and 12–13 of pregnancy. In turn, the PENK mRNA expression was increased by TNFα on days 10–11 and decreased by IL-1β on days 15–16 of pregnancy. The POMC expression was decreased by IL-1β on days 12–13, but increased by both IL-6 and TNFα on days 15–16 of the cycle. The PDYN mRNA expression in endometrial explants was increased by IL-1β on days 10–11 of the cycle. These results indicate that the opioid system is involved in the local regulation of uterine functions during the oestrous cycle end early pregnancy in pigs.