With a biological assay and atomic absorption spectrometry we determined the level of melarsoprol in the serum and cerebrospinal fluid of 19 patients treated with melarsoprol in Daloa, Ivory Coast. Most serum levels were between 2 and 4 micrograms/ml 24 h after administration, and were still > or = 0.1 microgram/ml after 120 h. Levels in the cerebrospinal fluid were between 0 and 0.1 microgram/ml. Elimination was biphasic, with a pronounced beta 1 phase. Mean terminal elimination half-life of melarsoprol was about 35 h, volume of distribution was about 100 l and total clearance was about 50 ml/min. The results of these first pharmacokinetic studies on melarsoprol were used to simulate possible alternative therapy schemes which might avoid some of the problems that arise with melarsoprol use.