To explore whether the difference in the amount of chloroquine (CQ) accumulated in CQ-resistant (CR) and in CQ-sensitive (CS) P. berghei is involved in the infected erythrocytes (RBC) or in the parasites.The concentration of CQ in infected RBC and parasites following ig administration of CQ to CS and CR infected mice was determined by HPLC method (extra standard).3 h after 4.06 mg/kg and 400 mg/kg administration, the accumulation of CQ in both infected RBC was similar, but in CR was 54.0% and 42.1% less than in CS (P < 0.001) respectively. The CQ accumulation in CS 3 h and 7 h after ig administration of CQ 400 mg/kg was similar (0.780 +/- 0.128 nmol/protein and 0.760 +/- 0.180 nmol/mg protein), while in CR the CQ-accumulation was higher at 7 h than that at 3 h after CQ administration (0.452 +/- 0.079 nmol/mg protein and 0.559 +/- 0.124 nmol/mg protein P < 0.02).The low CQ accumulation in CR is attributable to the parasite rather than to the infected RBC, and CR was not found to excrete CQ more rapidly than CS.