Prostaglandin synthetase inhibitors (eg, aspirin, indomethacin and naproxen) interfere with ovulation and prolong the gestation period in the rat. These properties could provide a novel test bed for the assessment of agents with prostaglandin biosynthesis inhibitory activity. Rats were fed green-lipped mussel extract and the effects on conception and the subsequent gestation period were determined. Parturition was delayed and fetal development was retarded in the Seatone-fed animals. The observations were consistent with the known effects of nonsteroidal anti-inflammatory drugs, suggesting that Seatone does contain pharmacologically active material inhibiting prostaglandin biosynthesis.