At therapeutic free concentrations (120-360 nM in rheumatoid arthritis), the accumulation ratio for chloroquine (7-chloro-4-[[4-(diethylamino)-1-methylbutyl]amino]-quinoline; CQ) in viable isolated rat hepatocytes is 795 +/- 33, which is of the same order of magnitude as in vivo hepatic uptake in the rat. The accumulation ratio is much lower in nonviable hepatocytes (12.4 +/- 0.5), showing that accumulation in membranes of hepatocytes accounts for a negligible proportion (less than 3%) of total accumulation at therapeutic free concentrations. This also indicates that the predominant mechanism of accumulation is dependent on structural integrity of cells and/or organelles. The accumulation ratio for CQ in viable hepatocytes is markedly reduced by NH4Cl and the metabolic inhibitors KCN and NaF. Since intralysosomal pH is known to be elevated in the presence of some weak bases (including NH4Cl and CQ) and metabolic inhibitors, this suggests that hepatic accumulation of CQ is a consequence of ion trapping in the acidic interior of lysosomes. Accumulation is linear at therapeutic free CQ concentrations; however, at CQ concentrations well above the therapeutic range, the accumulation ratio is markedly reduced. This is consistent with the known capacity of CQ to raise intralysosomal pH at these concentrations.