The influence of naproxen (a typical prostaglandin [PG] synthetase inhibitor) and arachidonic acid (AA) on the isolated uterus in estrus cycle (day 14) in guinea pigs was investigated. The results show that AA could increase the contractions of the uterus. Naproxen could inhibit the contractility caused by AA. The potency of inhibition was dose-dependent. The inhibition of tonus was quite obvious. The animal model is considered the suitable one for investigating the effect of PG synthetase inhibitor on myometrium.