We have investigated the ability of dopamine to compete with the binding of the high affinity dopamine D 2 receptor positron emission tomography (PET) radioligand, 18 F-fallypride. In vitro dissociation of 18 F-fallypride with dopamine in rat striatal homogenates exhibited a dissociation rate, k off , of 1.76 3 10 22 min 21 while the association rate constant, k on , was found to be 5.30 3 10 8 M 21 min 21 . This resulted in a dissociation constant, K D of 33 pM for 18 F-fallypride. For in vivo studies, we investigated the effects of reserpine and d-amphetamine treatment on 18 F-fallypride in an attempt to study competition of endogenous dopamine with the radioligand at the receptor sites in rats and monkeys. PET experiments with 18 F-fallypride in two male rhesus monkeys were carried out in a PETT VI scanner. In control experiments, rapid specific uptake of 18 F-fallypride in the striata was observed (0.05-0.06% injected dose (ID)/g) while nonspecifically bound tracer cleared from other parts of the brain. Striata/cerebellum ratios for 18 F-fallypride were approximately 8 at 80 min postinjection, respectively. The monkeys received various doses (0.25 to 1.50 mg/kg) of d-amphetamine (AMPH) pre-and postinjection of the radioligand. There was a decrease of specifically bound 18 F-fallypride as well as evidence of an enhanced clearance of specifically bound 18 F-fallypride after administering AMPH in the two monkeys. The dissociation rates, k off , of 18 F-fallypride without AMPH was ,10 24 min 21 but after 25 min preadministration of AMPH (1 mg/kg), it was 4.1 3 10 23 min 21 and after 17, 45 and 90 min postadministration of AMPH (1 mg/kg) it was 3.6 3 10 23 to 4.0 3 10 23 min 21 . Lower doses of AMPH (0.25 mg/kg) had a reduced effect on the binding of 18 F-fallypride. No effect was seen until about 30 minutes after the injection of AMPH. Studies with various doses indicated that 18 F-fallypride has a maximum response at doses of 0.75-1.50 mg/kg, with an approximately 16%/hour reduction in binding. These results indicate that AMPH stimulated release of endogenous dopamine reduces the specific binding of 18 F-fallypride.