Recent evidence suggests that decidualization can be considered a modified inflammatory reaction. In this work we intended to determine whether the prostaglandin inhibitor diclofenac could affect implantation. Rat blastocysts were cultured in diclofenac in vitro, then implanted to host mothers on day 5 of pseudopregnancy. Large doses of diclofenac in culture were toxic. Smaller doses had a profound effect on implantation. Another group of host mothers received diclofenac i.p., one hour prior to transfer of untreated blastocysts. The results were compared to parallel controls without diclofenac treatment. Control animals had a 72% implantation rate, whereas there was only a 35-41% implantation rate after in vitro diclofenac treatment. In the treated host mothers only 7% of embryos were normal, while 34% were growth-retarded. More normal embryos were found when the blastocysts were cultured with diclofenac. The observations indicate that diclofenac administered to the mother inhibits the ongoing process of implantation and placentation, whereas following in vitro exposure the embryo can recover from the insult. The implications for a better understanding of the process of implantation are discussed.