Quantitative measures of physiologic function with PET require continuous monitoring of arterial positron isotope concentration. A device has been developed that automates this process. This device has advantages over manual sampling techniques with syringes since fewer personnel are required, measurements are less error prone, and more continuous measures of arterial positron concentration are available. A constant flow infusion/withdrawal pump withdraws blood from the radial artery through a catheter connected to 0.5 mm inner diameter teflon tubing. This tubing is wrapped around a 50 mm thick by 50 mm diameter NaI(T1) crystal that is interfaced to a photomultiplier tube (PMT) and encased in a cylindrical lead shield. This crystal detects 511 Kev photons that result from positron annihilation. The device sensitivity is greater than 240 (cts/sec)/(microCi/ml) corresponding to a peak activity of approximately 10,000 cts/sec for a 50 mCi bolus injection in an adult. The system dynamic response has been measured and the true arterial input function is recovered by deconvolution. The system has been used clinically for more than 400 human PET studies and has been a reliable continuous monitor of arterial positron concentration.