Qinghaosu, also known as artemisinin and arteannuin, is a new type of antimalarial drug isolated from Artemisa annua L. Its low solubility in water and oil limited its widespread clinical use. Artesunate (sodium dihydroqinghaosu hydrogen hemisuccinate monoester) is easily soluble in water and is used iv in the treatment of acute cerebral and malignant malaria. However, artesunate was shown to have a very short half-life when given iv in animals as well as in human beings. A transdermal dosage form of artesunic acid had been prepared and was reported to have reliable suppressing and killing effects on plasmobium berghei in mice. This paper reports results of pharmacokinetic studies of this preparation when applied onto a fixed area of the shaved skin of mice and rabbits. Serum concentration of the drug was determined by a method of radioimmunoassay. The drug was found to be easily absorbed from the skin. The serum concentration-time curve is depicted in figures 1. Peak concentration of 1.8 micrograms/ml was reached at about 2 h when a dose of 25 mg/kg was given to rabbits. For mice, peak serum concentrations of 2.05 and 7.11 micrograms/ml were attained in about 0.5 h after doses of 31.3 and 71.4 mg/kg, respectively, while at a dose of 6.7 mg/kg a peak level of 0.82 micrograms/ml (a concentration more than 5000 times the IC50 of artesunate in in vitro tests on plasmodium berghei for antimalarial activity) was attained at about 4 h after application of the drug. The half-lives of the drug were found to be more than 2 h for both mice and rabbits.