In adult rats, using the single-pass brain uptake technique with a tritiated water standard, L-phenylalanine was shown to enter brain across the blood-brain barrier (BBB) by both a saturable and diffusionary process. A kinetic analysis of the data revealed a Michaelis constant (Ka) of 0.42 mM and a Vmax of 35 nmoles g-1 min-1. The saturable component of L-phenylalanine transport was adversely affected by dinitrophenol or or low sodium concentration in the intracarotid injection solution. Intravascular ouabain did not affect transport and only partial inhibition was observed with 1.5 mM L-leucine. It appears that there is no reginonal difference in the capacity of the BBB to move L-phenylalanine from blood to brain. This process is directly or indirectly coupled to sodium movement and is partially dependent on cellular energy.