Free diffusion appears to be the principal mechanism for movement of ions, known to concentrate in bone, across the capillaries of bone. The interstitial fluid space appears large enough to allow for determination of instantaneous fractional escape by the indicator-dilution method. The mechanism by which large molecules, such as the diphosphonate 99mTc-labeled EHDP or 99mTc-labeled pyrophosphate, pass through capillaries in bone is by passive diffusion. These molecules are larger than 85Sr and their lower extraction rate is presumably due to the effect of their sizes on passage through transcapillary clefts. A corollary of these studies might be that if transcapillary exchange is passive, then partition of anions and cations likely lies beyond the capillary and therefore is controlled by osteal cellular processes.