We analysed the effect of interleukin-1 on oocyte maturation and on steroid and prostaglandin production by equine granulosa and cumulus cells. In Experiment 1, interleukin-1β (IL-1β) was injected into the growing dominant follicle, which was punctured 38 h later. Follicular fluid was assayed for steroids and prostaglandin-F2α (PGF2α). Granulosa cells were analysed for 3β-hydroxysteroid dehydrogenase (3β-HSD), progesterone receptor (PR), cyclooxygenase 1 and 2 (Cox 1 and Cox 2) and steroidogenic acute regulatory protein (StAR) mRNAs. In Experiment 2, cumulus–oocyte complexes (COCs) were collected from slaughterhouse ovaries and cultured in different media: control group (TCM199 + BSA); Group 2 (+ IL-1β); Group 3 (+ EGF); Group 4 (+ EGF + IL-1β); and Group 5 (+ EGF + IL-1β + IL-1RA). Cumulus cells were analysed for 3β-HSD, PR, Cox 1, Cox 2 and StAR mRNAs. After injections of crude equine gonadotropin (CEG; LH effect) or IL-1β, progesterone and PGF2α levels increased, whereas 17β-oestradiol decreased. EGF induced an increase in the rate of in vitro maturation (P < 0.05), whereas IL-1β had a limited effect. IL-1β significantly decreased the rate of EGF-induced oocyte maturation (P < 0.05). Cox 2 mRNA level increases in granulosa cells after CEG injection (P = 0.07). In cumulus cells, StAR and PR mRNAs were lower in Group 2 and 3β-HSD mRNA was higher in Groups 4 and 5. These data confirm that IL-1 is involved in equine oocyte in vitro maturation. We demonstrated in vivo that IL-1β has an effect on steroids and PGF2α secretion in the preovulatory follicle.