1. 1. l-[ 14C]Leucine transport into Saccharomyces cerevisiae involves a high-affinity, low-velocity system (system 1) and a low-affinity, high-velocity system (system 2). These systems are characterized by the different values of the kinetic parameters K T and J max , and are both capable of concentrative transport. The general amino acid permease is assumed to be a part of the high-affinity system. 2. 2. The kinetics of l-[ 14C]leucine entrance show an initial rapid phase (the ‘very early uptake’) before reaching the steady-state rate. The contribution of the very early uptake to total entrance values affects the values of K T and J max , especially when the steady-state rate is relatively slow, as with starved yeast, and then negative K T and J max values may result. The very early uptake is increased by pretreatment of starved yeast with d-glucose, this latter effect being counteracted by iodoacetate. 3. 3. After energization of starved yeast by pretreatment with d-glucose or propionaldehyde, the apparent K T,2 value greatly decreases whilst the K T,1 value decreases to a much more limited extent, or does not vary. With the energized yeast, K T,2 decreases throughout incubation whilst K T,1 variation is insignificant. Energization increases J max,1 and J max,2 several-fold and with the energized yeast at the steady-state phase, J max,2 ⩾ 4J max,1 . Variation of K T and J max values as a function of the metabolic state of yeast cells may be explained in terms of variation of rate constants k −1, k +1 and k +2 for each transport system. 4. 4. Dicyclohexylcarbodiimide, quercetin and diethylstilbestrol inhibit transport at 0.05 mM l-[ 14C]leucine, in good agreement with a function of the plasmalemma ATPase for the operation of system 1. Dio-9, propionic and isobutyric acids, pentachlorophenol, carbonylcyanide 3-chlorophenylhydrazone and carbonylcyanide 4-trifluoromethoxyphenylhydrazone, which affect the proton gradient and/or the membrane potential inhibit l-[ 14C]leucine uptake at all the assayed amino acid concentrations. 5. 5. The polyene antibiotic, nystatin, which forms channels in membranes permeable to K + and H +, inhibits systems 1 and 2 activity but enniatin (also a K + ionophore) does not.