A growing body of evidence suggests that the ovary is a site of inflammatory reactions, and thus, ovarian cells could represent sources and targets of the interleukin-1 (IL-1) system. The aim of the present work was to investigate the expression of IL-1α, IL-1β, IL-1ra, IL-1R1, and IL-1R2 genes in equine cumulus cells and oocytes. Moreover, the influence of IL-1β on in vitro maturation of cumulus-oocytes complexes (COCs) was examined. COCs were collected using ultrasound-guided follicular puncture in vivo. Oocytes and cumulus cells were isolated from preovulatory and subordinate follicles and were analyzed either at collection or after in vitro culture. An ultrasensitive reverse transcription-polymerase chain reaction was used to study the expression of IL-1 system members. In order to study the in vitro effect of IL-1β on oocyte nuclear maturation and cumulus expansion, immature COCs collected from subordinate follicles were cultured for 30 h in media containing or not containing IL-1β alone, or in combination with equine luteinizing hormone. Our results indicated that equine oocytes expressed IL-1β and IL-1R2 genes, and that cumulus cells expressed all IL-1 system members except IL-1α. In oocytes, IL-1β expression significantly decreased during in vitro culture compared with immature and mature oocytes analyzed at collection. Similarly, in cumulus cells, in vitro culture decreased IL-1ra and IL-1R1 expression. IL-1β was shown to significantly decrease the gonadotropin-induced nuclear maturation of oocytes. In conclusion, we demonstrated for the first time, in mares, the presence of IL-1β and IL-1R2 transcripts in oocytes and of IL-1β, IL-1ra, IL-1R1, and IL-1R2 in cumulus cells. The regulatory effect of IL-1β on COCs during maturation suggests that the IL-1 system could be of crucial importance for ovarian physiology in the mare.