A miniature scintillation β microprobe has been developed to measure the input function in live rodents for use in longitudinal, quantitative PET studies. The probe consists of a small lutetium oxyorthosilicate (LSO) crystal measuring typically 0.3-0.6 mm diameter x 0.5-2 mm in length that is used to directly detect positrons in the blood or tissue. The probe has a sensitivity of 10-40 Hz/μCi/cc and is primarily sensitive to short range positrons emitted by labeled radiotracers in the blood. The sensitivity to gamma-ray background can be minimized using a variable threshold in the readout to discriminate between positrons and gammas. The probe was implanted in one of the tail veins of a Sprague-Dawley rat and the input function was measured for the injection of 0.8 mCi of FDG in the other tail vein. The probe exhibits a fast time response that is able to quickly and accurately measure the concentration of 18F circulating in the bloodstream. Additional tests were also carried out to study the probe's sensitivity to gamma ray background.