Pentamidine, recently released for clinical use, is effective in therapy for the hemolymphatic stage of Gambian trypanosomiasis, antimony-resistant leishmaniasis, and Pneumocystis carinii pneumonia. The mechanism of action is unclear and may differ for different organisms. Trypanosomes actively transport pentamidine intracellularly, and the drug may then interfere with DNA biosynthetics. However, pentamidine appears to kill nonreplicating P. carinii. The mechanism of killing is unexplained. The pharmacokinetics of pentamidine has been incompletely studied in humans. The estimated volume of distribution is 3 liters/kg. Levels in plasma of pentamidine range from 0.3-1.4 microgram/ml after standard 4 mg/kg dosing, with no appreciable increase in drug levels on successive dosing and no correlation between levels and creatinine clearance or adverse reactions. The drug appears to be concentrated in the kidney and excreted in the urine, with levels detectable six to eight weeks after cessation of therapy. Immediate adverse reactions have included hypotension, nausea, and vomiting. Local pain or abscess formation at an injection site, mild azotemia, leukopenia, abnormal findings from liver function tests, and hypoglycemia may also occur.