Picralima nitida, an Apocynaceae plant distributed in West Africa, has seeds used in traditional medicine including for malaria, though a previous study found no effect on avian malaria. We evaluated the antimalarial activity of its seed extracts and fractions against Plasmodium falciparum strains D6 (chloroquine-sensitive) and W2 (chloroquine-resistant). Seeds from Ghana were defatted and extracted to obtain various fractions. Most fractions showed ED50 >3300 ng/ml, but the phenolic alkaloid fraction (Pic-10Ph) had ED50 830 ng/ml (D6) and 1760 ng/ml (W2). Akuammine, the main phenolic alkaloid, exhibited ED50 530 ng/ml (D6) and 1110 ng/ml (W2). No toxicity to mammalian cells was observed at 20,000 ng/ml, indicating selective antimalarial effect; Rhazya stricta, a small glaborous shrub abundant in Pakistan and used in indigenous medicine, was studied. We identified rhazimine, isolated from the plant, as a dual inhibitor of arachidonic acid metabolism and platelet activating factor (PAF)-induced platelet aggregation.