Fourteen compounds isolated from Stephania venosa, Prismatomeris sessilifloro, Diospyros montana and Murraya siamensis were tested for their antimalarial potential. The 6a,7-dehydroaporphine alkaloids dehydrostephanine and dehydrocrebanine showed potent activity with IC50 values of 40 and 70 ng/ml, respectively. The C-13-NMR data of rubiadin, rubiadin-1-methyl ether, diospyrin and 5-hydroxy-4-methoxy-2-naphthaldehyde were extensively studied.