We have applied elastic-scattering spectroscopy (ESS) for noninvasive, real-time in vivo measurement of the concentrations of certain drugs in tissue, utilizing a simple fiber-optic-probe spectroscopic system. The system uses a broadband light source, enabling the detection of compounds with absorption bands in most regions of the visible, as well as the NIR to 1700 nm. Subcutaneous tumors were grown in 4 Nude mice; the mice were treated with one of two chemotherapy agents, and the ESS system was used to perform pharmacokinetics measurements on the tumors following drug administration. Time histories of the drug concentrations in the tumors agreed with the known pharmacokinetics of the two drugs, and HPLC assays following sacrifice showed good correlation with the ESS values. Most photodynamic therapy agents and many chemotherapy drugs, including any that are not fluorescent, are ideal candidates for the ESS system. The measurement can be calibrated absolutely, and is not susceptible to problems associated with fluorescence assay methods.