SummaryAn experiment was undertaken to determine whether the equine conceptus inhibits production of prostaglandin F2αin the pregnant mare. Endometrial tissue was obtained from two normally cycling mares and conceptus membranes were obtained from a further two mares which were pregnant. Endometrial tissue was cultured alone and with conceptus membranes and the prostaglandin F2αconcentrations measured in the incubation medium over a 24 h period. With endometrium alone, prostaglandin F2αconcentrations increased slowly for the first 9 h of incubation but then began to rise sharply. When the conceptus membranes were added, the prostaglandin F2αproduction rate was initially comparable to that of endometrium alone but the sharp increase seen after 9 h was large attenuated. By 18 to 24 h, the prostaglandin F2αconcentration in the endometrium plus conceptus membranes was 63 per cent less than for endometrium alone. The results corroborate previous studies that conceptus membranes can inhibit prostaglandin F2αsynt