Qinghaosu (QHS), also known as artemisinin and arteannuin, is a novel type of sesquiterpene with a peroxide linkage isolated from the Chinese herb Artemisia annua L. Since its discovery as an antimalarial with low toxicity, hundreds of derivatives have been synthesized among them artesunate (ATS), artemether (ATM) and dihydroqinghaosu (DHQHS) were found to be more active than QHS itself. A suppository of QHS, a dual-pack dosage form of ATS (artesunic acid to be dissolved in sodium bicarbonate solution just before iv injection) and an oil solution of ATM for im injection had been approved by our Ministry of Health for clinical use. However, a preparation for oral administration is still not available. We have reported that when dogs were given QHS tablets orally at the dose of 70 mg/kg, no drug was detected in the serum using the RIA method, whereas appreciable serum concentration was found by the same method when dogs were given DHQHS tablets at a dose as low as 10 mg/kg. This paper reports the pharmacokinetics of DHQHS in man studied with the RIA method and compared with QHS. When DHQHS in tablet form was given to human volunteers at doses of 1.1-2.2 mg/kg, peak serum levels of 0.13-0.71 micrograms/ml were obtained in 1.33 h with MRT of 2.26-2.36 h. When QHS tablets were given at the dose as high as 15 mg/kg, however, the peak serum level found in 1.5 h was only 0.09 microgram/ml with MRT of 1.33 h. Therefore, the bioavailability of QHS tablets is only 1.62-10.08% that of DHQHS.(ABSTRACT TRUNCATED AT 250 WORDS)