A high-performance liquid chromatography (HPLC) method for the simultaneous determination of mefloquine and chloroquine in biological fluids is described. Methaqualone is used as an internal standard. After a single-step extraction, achieved from alkalinized samples with diethyl ether, a separation is obtained using a mu-Bondapak phenyl column with a mobile phase of acetonitrile and sodium acetate buffer. Detection is done at 305 nm. The lower detection limits are 3 and 10 micrograms/l for mefloquine and chloroquine, respectively. Thus, this method appears to be simple, rapid, very sensitive and applicable in pharmaco-clinical, toxicological and forensic analyses.