Dihydroorotase and dihydroorotate dehydrogenase, two enzymes of the pyrimidine biosynthetic pathway, were purified from Plasmodium berghei to apparent homogeneity. Orotate and a series of 5-substituted derivatives were found to inhibit competitively the purified enzymes from the malaria parasite. The order of effectiveness as inhibitors on pyrimidine ring cleavage reaction for dihydroorotase was 5-fluoro orotate greater than 5-amino orotate, 5-methyl orotate greater than orotate greater than 5-bromo orotate greater than 5-iodo orotate with Ki values of 65, 142, 166, 860, 2200 and greater than 3500 microM, respectively. 5-Fluoro orotate and orotate were the most effective inhibitors for dihydroorotate dehydrogenase. In vitro, 5-fluoro orotate and 5-amino orotate caused 50% inhibition of the growth of P. falciparum at concentrations of 10 nM and 1 microM, respectively. In mice infected with P. berghei, these two orotate analogs at a dose of 25 mg/kg body weight eliminated parasitemia after a 4-day treatment, an effect comparable to that of the same dose of chloroquine. The infected mice treated with 5-fluoro orotate at a lower dose of 2.5 mg/kg had a 95% reduction in parasitemia. The effects of the more potent compounds tested in combination with inhibitors of other enzymes of this pathway on P. falciparum in vitro and P. berghei in vivo are currently under investigation. These results suggest that the pyrimidine biosynthetic pathway in the malarial parasite may be a target for the design of antimalarial drugs.