Luteinizing hormone releasing hormone (LHRH)-like immunoactivity has been identified in cotyledonary extracts prepared from pregnant ewes. This activity displayed similar physico-chemical properties as synthetic LHRH, as determined by reverse-phase HPLC and size-exclusion HPLC. Under reverse-phase conditions, cotyledonary LHRH-like immunoactivity displayed a retention time (10.5 +/- 0.1 min) which was not significantly different from that of synthetic LHRH. When subjected to size-exclusion HPLC, cotyledonary LHRH-like immunoactivity eluted in fractions which corresponded to a molecular weight range of 1100-1200 Da, which was not significantly different from the elution profile observed for synthetic LHRH. The cotyledonary tissue content of LHRH-like immunoactivity averaged 94 +/- 24 pg/mg (n = 6). The results of this study demonstrate the presence of LHRH-like immunoactivity in ovine cotyledons. Although placental synthesis of LHRH-like immunoactive material has been demonstrated in other species, it remains to be established whether this activity, demonstrated in ovine placenta, is the consequence of de novo placental synthesis or represents uptake from the maternal (and/or fetal) circulation.