Dopamine D2-receptor density in striatum of monkey was measured with 3-(2'-[18F]fluoroethyl)spiperone (FESP) and dynamic positron emission tomography (PET), using a double-injection technique. A first bolus of high specific activity (SA) FESP (5 mCi; approximately equal to 1 Ci/mumol) was injected i.v.; 90 min later, a second bolus of lower SA FESP (5 mCi; approximately equal to 0.04 Ci/mumol) was injected. A dynamic PET study was performed to measure the kinetics of FESP in striatum over 180 min, and the metabolite-corrected concentration of FESP in plasma as a function of time was obtained from arterial blood samples. A nonlinear compartmental model that took into account the saturability of the receptor binding was used to describe the kinetics of FESP in striatum. Model parameters were estimated by regression with a constraint based on information about the equilibrium dissociation constant of the ligand-receptor binding. Dopamine D2-receptor density in striatum was estimated to be 25.9 +/- 12.7 pmol/g in seven Macaca nemestrina monkeys. The method does not require the use of cerebellum as a reference tissue region and an estimate of dopamine D2-receptor density can be obtained from a single study.