The evaluation of every new radiotracer involves pharmacoki-netic studies on small animals to determine its biodistribution and local kinetics. To extract relevant biochemical information, time–activity curves for the regions of interest are mathematically modeled on the basis of compartmental models that require knowledge of the time course of the tracer concentration in plasma. Such a time–activity curve, usually termed input function, is determined in small animals by repeated blood sampling and subsequent counting in a well counter. The aim of the present work was to propose an alternative to blood sampling in small animals, since this procedure is labor intensive, exposes the staff to radiation, and leads to an important loss of blood, which affects hematologic parameters. Methods: Monte Carlo simulations were performed to evaluate the feasibility of measuring the arterial input function using a positron-sensitive microprobe placed in the femoral artery of a rat. The simulation results showed that a second probe inserted above the artery was necessary to allow proper subtraction of the background signal arising from tracer accumulation in surrounding tissues. This approach was then validated in vivo in 5 anesthetized rats. In a second set of experiments, on 3 rats, a third probe was used to simultaneously determine 18 F-FDG accumulation in the striatum. Results: The high temporal resolution of the technique allowed accurate determination of the input function peak after bolus injection of 18 F-FDG. Quantitative input functions were obtained after normalization of the arterial time–activity curve for a late blood sample. In the second set of experiments, com-partmental modeling was achieved using either the blood samples or the microprobe data as the input function, and similar kinetic constants were found in both cases. Conclusion: Although direct quantification proved difficult, the microprobe allowed accurate measurement of arterial input function with a high temporal resolution and no blood loss. The technique, because offering adequate sensitivity and temporal resolution for kinetic measurements of radiotracers in the blood compartment , should facilitate quantitative modeling for radiotracer studies in small animals.