A rapid method for the determination of DHT (5 alpha-dihydrotestosterone) in serum was established, utilizing potassium permanganate as an oxidising agent to convert the double bond at position 4-5 of testosterone to dihydroxy alcohols. DHT was then extracted with diethyl ether and measured directly using radioimmunoassay technique. The precision of the assay was determined at three levels of DHT added into pooled serum and found to be 4.4-5.1% for between assay percentage coefficient of variation. The sensitivity of the standard curve was found to be 4.2 pg/0.5 ml sample. The accuracy was determined in pooled serum with added standard DHT in the range of 80-800 pg. The percentage recovery of the assay was found to be 92.93-98.35%. The antisera used for DHT assay exhibited no significant cross-reaction with various steroids tested except testosterone which showed a 91.3% cross-reaction. The method was applied to measure the level of DHT in serum of Thai males receiving medroxyprogesterone acetate (MPA) as an oral contraceptive. Twenty-five subjects were divided into 5 groups. Group 1 received an oral placebo with monthly sesame oil injection. Groups 2, 3 and 4 received 5 mg, 10 mg and 20 mg MPA, respectively, with the addition of monthly injection of 250 mg testosterone enanthate (TE). Group 5 received 20 mg MPA daily with the addition of monthly injection of 500 mg TE. Blood specimens were obtained monthly. The first 4 months were used as a control period. After the 4-month period, each subject received treatment as mentioned above for 6 months, then follow-up study was resumed for another 6 months. The results revealed that subjects receiving 20 mg MPA with the addition of 250 mg TE showed a significant decrease of DHT levels (p less than 0.001).