Inhibition of microsomal prostaglandin (PG) biosynthesis by allantoic fluid, obtained from ewes at 80–120 days of gestation, was examined. Inhibition of cotyledonary microsomal PGE2 and PGF2α biosynthesis by lyophilized allantoic fluid occurred in a dosedependent manner. The concentration of allantoic fluid required to inhibit PGE2 and PGF2α production by 50% averaged 17·9 ± 3·2 (s.e.m.) mg dry weight/ml (n = 5). Microsomal PG biosynthesis was markedly enhanced by the addition of arachidonic acid (30 μmol/l). Synthesis of PGE2 and PGF2α was increased to 245 ± 65% and 184±14% of control (P<0·05, n = 5) respectively. Treatment of cotyledonary microsomes with porcine phospholipase A2 (PLA2; 0·125 units/ml) also stimulated PG synthesis, PGE2 increasing to 216 ± 27% and PGF2α to 172 ± 14% of control (P<0·05, n=5) respectively. Allantoic fluid (20 mg dry weight/ml) inhibited arachidonic acid-stimulated PG synthesis (PGE2 by 48·6 ± 13·8% and PGF2α by 44·2 ± 7·7%) and PLA2-stimulated PG synthesis (PGE2 by 60·6±11·6% and PGF2α by 74·8 ± 8·5%). Allantoic fluid, however, did not affect PLA2-stimulated release of arachidonic acid from microsomes, thus negating the possibility that allantoic fluid suppresses PG synthesis by inhibiting PLA2 activity. These data indicate that allantoic fluid inhibits PG production at the level of PG synthase enzymes. Prostaglandin inhibitor(s) in allantoic fluid may play a role in maintaining uterine quiescence throughout gestation and its withdrawal, at term, may be involved in the initiation of labour.