The kinetics of exchange of strontium (85Sr) and potassium (42K) were studied in the mid-tibial cortical bone of 37 adult dogs. After injection of these two tracer cations and tracer-labeled albumin into the tibial nutrient artery, two types of observations were made: 1) collection of sequential venous samples to provide the outflow indicator-dilution curves and to calculate the extraction and retention at early times; and 2) detection of energy-selected gamma emissions via a detector over the tibia to give the time course of content of 42K and 85Sr in the tibia. Extractions of K+ and Sr2+ were 50 and 60% during a single transcapillary passage. More Sr2+ than K+ was retained in the first minutes. Their rates of washout over a 3-h period were similar. The interpretation is that the rate of uptake at extravascular sites is faster for Sr2+ than for K+, as is the rate of release, and that the extravascular volume of distribution for Sr2+ (adsorption sites in the interstitium or on bone) is much larger than that for K+ (intracellular).