This study quantifies the parenchymal and nonparen-chymal uptake of technetium-99m (99mTc)- and indium-III (111In)-low-density lipoprotein (LDL) in different states of hepatic LDL-receptor activity to validate quantitative LDL scintigraphy. Iodine-125 (125I)-LDL was used as reference tracer. Four Sprague-Dawley rats with 17-alpha-ethinyl estradiol (EE)-stimulated LDL-receptor activity and five controls received all three tracers simultaneously 90 minutes before collagenase liver perfusion and metrizamide gradient cell separation. Total liver uptake of 99mTc-, 111In-, and 125I-LDL was 1.8 ± 1.0, 1.6 ± 0.8, and 0.2 ± 0.2% injected dose/g organ weight, respectively. The contribution of nonparenchymal cells to total hepatic tracer uptake was 5.4 ± 4.7%, 11.6 ± 10.3%, and 9.6 ± 7.6% in controls. Estradiol treatment increased total liver uptake to 2.4 ± 0.5, 2.0 ± 0.2, and 0.5 ± 0.3% injected dose/g and reduced nonparenchymal cell contribution to 2.3 ± 2.6%, 4.2 ± 4.8%, and 2.6 ± 2.9%, respectively. Dual-isotope scintigraphy in EE-treated and control rats confirmed these data, with a lower total hepatic uptake of 111In-LDL in comparison with 99mTc-LDL but a comparative degree of increase by EE treatment. Both behave quantitatively comparable as residu-alizing tracers, yet 99mTc-LDL shows a higher affinity to the LDL receptor pathway of parenchymal cells. However, the nonspecific uptake of both tracers can be neglected for quantitative LDL scintigraphy, and external imaging of hepatic tracer uptake primarily reflects LDL-receptor activity of parenchymal cells. (HEPATOLOGY 1995; 22:1289–1295.).