Abstract: The contents of free amino acids and the activity of ornithine decarboxylase in infected RBC of chloroquine-sensitive strain of Plasmodium berghei(CS) were similar to those in chloroquine-resistant strain (CR). After treatment with chloroquine (10mp/kg im) no effect on the formation of free amino acids was found after 20th, but chloroquine (5mg/kg im) inhibited the enzymatic activity of the CS RBC by 79.6% whereas that of the CR by 55.7%.The contents of spermidine in CS and CR infected RBC were found to be 139 ±27 and 528±140 nmol/109 infected respectively. Cycloleucine inhibited the formation of spermidine by 44% in CS and 57% in CR groups, while co-administration of methionine (100mg/kg) with cycloleucine (80mg/kg) could reverse the inhibitory effect and raise the contants of spermidine to 294 ± 56 and 657±129 nmol/109 infected RBC in CS and CR groups respectively. This indicates that methionine had significant effect on the formation of spsrmidine in CS strain, but there was still a remarkable difference, when compared with CR strain. It is suggested that the activities of S-adenosyl-methionine synthetase and other enzymes involved in polyamine formation might be the causal factors.