The kinetic data of two different 99mTc-Sn-pyrophosphate compounds (compound A and B) were evaluated in non-adult rats. Only compound A concentrated in bone. Both compounds dispersed rapidly in the intravascular as well as the extravascular space. The plasma protein bond of both compounds increased with time after injection and impaired both the renal clearance of both compounds and the bone clearance of compound A. The renal clearance of both compounds was somewhat above that of 51Cr-EDTA. It is concluded that compound A and B is mainly excreted by glomerular filtration. About one fourth of the glomerular filtrate of compound B is reabsorbed and accumulated by the tubular cells.