Objective-To compare the effects of various IUDs on intrauterine activity. Patients and Methods-57 women volunteers aged 18 to 25 in the outpatient clinic of the Department of Obstetrics and Gynecology at Cerrahpasa Medical Faculty of Istanbul University were included in the study. All were in good health and menstruating normally. The subjects were introduced to different types of IUDs on the fifth to eighth days of their menstrual cycle. Intrauterine activities with and without naproxen were recorded, for at least 20 minutes-each time, with a micro-tip transducer before insertion, lust after insertion, and 1 month later. Results-Intrauterine activity increased lust after insertion of both IUDs and disappeared 1 month later in copper-T wearing women, but persisted in multiload wearers. Naproxen seemed to be useful in preventing intrauterine activity caused by the IUD. Conclusions-The IUD itself causes an increase in intrauterine activity which is preventable by naproxen, suggesting the possible role of prostaglandins. Persistence of intrauterine activity depends upon the type of device. Optimal designing of IUDs is required to decrease side effects and make them more acceptable.