Indicator dilution experiments were done to determine the extraction of 85Sr during a single passage through capillaries of the tibial diaphysis. Extraction was estimated by injection of 85SrCl2 and a nonpermeant, reference tracer, T-1824-labeled albumin, into the nutrient artery and recording of the effluent venous dilution curves (femoral vein). The mean (+/- SD) maximal instantaneous extraction was 0.53 +/- 0.08 (N = 12). Net retention after 10 min, estimated from venous curves, was 0.41 +/- 0.06 (N = 12), which appeared not substantially different from the retention estimated by direct isotope counting of the tibias for 85Sr, 0.35 +/- 0.06 (N = 12). In a second set of experiments in intact animals, tibial 85Sr extraction after intravenous injection was apparently higher, 0.53 +/- 0.28 (N = 15). Values of tibial diaphyseal blood flow, estimated from washout curves for iodoantipyrine after tibial nutrient artery injection, were 1.47 +/- 0.63 ml/min per 100 g (N = 27). The extraction was not much diminished by higher flows. The estimates of permeability-surface area product (PS) for bone capillaries did increase with flow, suggesting recruitment of more capillaries at higher flows. PS values averaged 0.63 +/- 0.29 (N = 12); we conclude that the capillary membrane is a primary barrier to the passage of 85Sr and presumably other small hydrophilic solutes.