Alendronate and etidronate are bisphosphonates used clinically to treat diseases associated with increased bone resorption. Etidronate is less potent and was reported to cause osteomalacia. This study examines if differences in distribution of alendronate and etidronate in the skeleton can explain differences in efficacy and in effects on mineralization between the two drugs. Eight-day old rat pups were injected s.c. with 3H-alendronate or 3H-etidronate both at either 1.3 mumol/kg or at their respective pharmacological effective doses in the growing rat of 0.12 mumol/kg for alendronate and 72.8 mumol/kg for etidronate. Twelve hours after administration at 1.3 mumol/kg both drugs showed a three- to fourfold higher localization on osteoclast vs. osteoblast surface. At the pharmacologically effective doses, 3H-alendronate labeled eightfold more osteoclast surface than osteoblast surface. In contrast, 3H-etidronate labeled approximately equal fractions of osteoclast and osteoblast surface. When similar doses of 3H-etidronate and 3H-alendronate (0.24 mumol/kg 3H-etidronate vs. 0.20 mumol/kg 3H-alendronate; 1.5 mumol/kg 3H-etidronate vs. 1.2 mumol/kg 3H-alendronate; and 14.6 mumol/kg 3H-etidronate vs. 12.0 mumol/kg 3 H-alendronate) were injected intravenously into adult mice at similar specific activities, 3H-etidronate labeled 1.5-2.5 times more osteoclast surface than 3-H-alendronate, but 3 to 15 times more osteoblast surface. Consequently, the ratio between the fraction of labeled osteoclast surface and the fraction of labeled osteoblast surface ranged for 3H-alendronate from 9 to 24, whereas for 3H-etidronate the range was from 4 to 7, due to more extensive labeling of osteoblast surface by 3H-etidronate. In a third experiment, we confirmed in adult mice the previous observation made in rat pups that normal bone formation occurs over alendronate-covered bone surfaces, and found that it occurred over etidronate-covered surfaces as well. Forty nine days after s.c. administration of alendronate at 0.12 mumol/kg or etidronate at 1.3 mumol/kg or 55.3 mumol/kg into adult mice bone formed over drug label. The distance from incorporated label to bone surface for both drugs (12.7 microns for alendronate and 8.7 and 9.2 microns for etidronate) was similar to wall width (defined by cement line) in controls (10.6 microns). In conclusion, alendronate, especially at pharmacologically active doses, shows higher uptake on resorption vs. formation surfaces than etidronate. The extent of bone formation on surfaces containing alendronate or etidronate is similar and is comparable to the "wall width" in controls.