The erythrocytic stages of Plasmodium yoelii (BY 265 strain) were incubated with nitroquine (0.2 pmol.L-1 - 20 mumol.L-1), cycloheximide (0.1-10 nmol.L-1) and aminopterin (0.2 pmol.L-1 - 20 mumol.L-1) during 8 h in vitro. The results showed that the incorporation of [3H]-isoleucine into the protein by Plasmodium yoelii was inhibited by nitroquine (0.2 pmol.L-1) and cycloheximide (0.1 nmol.L-1), the inhibitory effect of nitroquine (IC50 = 0.16 nmol.L-1) was 397 times higher than that of cycloheximide (IC50 = 63.5 nmol.L-1). However, the incorporation of [3H]isoleucine into protein was not affected during at least 8 h of exposure to aminopterin (20 mumol.L-1). The results suggest that the inhibition of protein synthesis by nitroquine is one of the mechanisms of the early antimalarial action.