A technique is described which uses x-ray fluorescence to determine in situ the strontium content of bone. Measurements were made of the skulls of seven rabbits both before and after a single bolus injection of strontium was administered intravenously. The retention of strontium was followed for up to eight months and could be represented either by a three-exponential model or, for data obtained between 12 and 252 d following injection, the power function At-b, where t is the time in days following injection. The three-exponential terms had linear coefficients of similar magnitude and half lives of 3-4 h, 10-40 d and 200-300 d. The exponent of the power function was 0.25-0.57. Measurements were also made of the skull and medial surface of the tibial crest of an adult dog for four months following subcutaneous administration of strontium. The results obtained using x-ray fluorescence are found to be similar to those reported by others for radiostrontium studies. The precision and errors associated with the fluorescence technique are discussed. Estimates of radiation dose are given. It is concluded that the technique may serve to determine in situ the strontium content of bone near the surface of the body.