The in situ brain perfusion technique of Takasato et al. allows accurate measurements of regional amino-acid influx across the blood-brain barrier. Influx can be determined in the absence of competing amino acids, using saline perfusate, or in the presence of physiological concentrations of amino acids, using plasma or blood perfusate. The concentration dependence of cerebrovascular LNAA transport is best described by a model with a Michaelis-Menten saturable and a nonsaturable component. Vmax values for four LNAAs are similar and average 10 X 10(-4) mumol X sec-1 X g-1. In contrast, Km values differ by up to 10-fold with the lowest value for leucine. Competitive inhibition increases the apparent Km for transport from plasma by approximately 20-fold. These data provide accurate new estimates of the kinetic constants that describe amino-acid transport across the blood-brain barrier. In addition, they indicate that the cerebrovascular transport capacity (Vmax) and affinity (1/Km) are greater than previously reported with the brain uptake index technique.