We used a simplified probe detection system for positron-emitting radionuclides in order to measure blood-brain barrier transport of amino acids in anesthetized dogs. Plasma and brain time-activity curves were recorded after intravenous bolus injection of L-[11C]methionine before and after administration of 1 microgram of vasopressin. Three-compartment models with three or four transfer coefficients were used to derive the kinetics of L-[11C]methionine uptake in brain. The blood-brain clearance of the tracer (K1) was 0.075 ml ml-1 min-1 before and 0.041 ml ml-1 min-1 after injection of vasopressin. The partition volume and the initial distribution (plasma) volume of methionine were unchanged and within the expected limits. The net accumulation rate of methionine (K), estimated by both the four-parameter (kinetic) and three-parameter (graphic) approaches, decreased after vasopressin injection in all six studies.