The pharmacological treatment of migraine may be acute or preventive. Frequent, Severe, complicated and long lasting migraine attacks, more than 2 attacks in month require prophylaxis. Traditional preventive drugs such as ?-Blockers, antidepressants, calcium blockers and anticonvulsants, despite their documented efficacy, have many side effects. This study was conducted to determine the effect of enalapril in prevention of migraine headache attacks. Seventy patients aged 19 to 59, who had migraine headache attacks, two to six times in month for at least one month were studied. They were randomly divided into two groups. One group received 5mg enalapril tablet twice a day (totally 10 mg per day) and the second group received 200 mg sodium valproate tablet twice a day (totally 400 mg per day). Both groups received the drugs for eight weeks. At the end of each month all the patients were asked about the number and the duration of headache attacks, accompanying complaints, and analgesic use during the study period. In enalapril group, total number of migraine headaches reduced to 2-3 times per month after two month totally, 42.8 percent of the enalapril group experienced two attacks and 48.5 percent experienced six attacks per month. In valproate sodium group 20 percent had four attacks and 8.5 percent experienced six attacks per month. The total number of migraine attacks, analgesic use and photophobia were statistically lower in enalapril group (P<0.001). The results of this study reveal that enalapril is more effective than valproate sodium in prevention of migraine headache attacks. We suggest that the efficacy of long period prevention with enalapril (more than 2 month) deserves further investigation.