The present study has developed a now HPLC techique which is sensutive enough lo detect the enzyme dihydroorotate dohydrogonase (DH0 DH) In both asexual Intraerythrocylic stage and sexual ( garmetocyle > stage of the human malaria parasile, P. falciparum The column used was C(,18) mBondapak reverse phase. The elution was isocralic with 3 mM PIC A in 5 mM ammonium dhydrogenphosphate containing 5% methanol. This condition was determined from the appropriate capacity faclor and response factor of each component. Under these conditions the substrate, dihydroorotic acid and the products, orotic acid and UMP were clearly separated. It could be demonstrated that the increasing amount of the product ( orolic acid ) was coincident with the decreasing amount of the substrate < dihydroorotic acid >. Additionally, it also clearly demonstrated that the detected DHO-DH was of parasite origin since there was a linear relationship between the amount of parasite protein, the number of parasites and the percent parasitemia. The intact parasile prepared directly by saponin Iysis of parasitized red cells showed The highest enzyme activity compared lo sonication and freeze-thawing of the parasites. Using the developed HPLC method, DHO-DH activity could be demonstrated in all intraerythrocytic slages of P. falciparum. The ring stage had the lowest activity while the schizonl had the highest activity. The DHO-DH activity was reported for the first time in preparations of the gamelocyles frorn culture of P. falciparum which showed the level of the enzyme was approximately that found in the asexual trophozoite stage. This new HPLC technique was also used to study the inhibition of pyrimetharnine, primaquine, and WR 238605, a novel primaquine derivative, as well as menoclone, a milochondrial DHO-DH inhibilor, on parasite DHO-DH al 4h and 24 h of drug exposure. Menoctone was observed lo inhibit parasite enzyme synthesis in the absence of parasite killing. Pyrimethamine and prirmaquine have no effect on parasite DHO-DH inhibition while WR 238605 has partial inhibition on the parasite enzyme. This conclusion was also applied to the gametocyte culture at 4h drug exposure except primaquine still showed partially effect. In addition to the evalution of the above conclusion by the determination of parasite DHO-DH, fluorescent activated cell sorting (FACS) was also performed. FACS showed a nice linear correlation in determination of parasitemia to the conventional Clomsa staining thin smears. FACS could also determine the percent parasitemia when the parasites were at the ring stage but with the lower fluorescent intensity compared to the more mature stage. Therefore, the mean values of the fluorescent intensity of the parasitized cells showed the clear interpretation while the percent parasitemia showed no difference in the demonstration of antimalarials and menoctone. Thus, it needs Giemsas staining smears for the identification of the stage of parasite.