A miniature scintillation 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 0.3-0.5 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. This probe is shorter in length which makes the placement easier.