1. 1. The uptake of α-aminoisobutyric acid (AIB) was measured in Ehrlich ascites tumor cells which were treated by osmotic shock and cold incubation to elevate cell Na+ and lower K+. These cells were suspended in media in which Na+ was partially replaced by choline or K+, thereby reversing the Na+ concentration gradient or both the Na+ and K+ concentration gradients. 2. 2. There was an inward transport of AIB against its concentration gradient when the Na+ concentration gradient was reversed and also when both the Na+ and K+ concentration gradients were reversed. 3. 3. In other experiments, the steady-state AIB and ion distribution ratios were measured, and the energy expenditure needed to maintain the AIB concentration gradient was calculated and compared with the energy expenditure calculated to be available from the ion electrochemical potential gradients in two model carrier systems. 4. 4. A model in which there was a 1:1 cotransport of Na+ and AIB and no associated K+ movement was found to be inadequate to explain the entire AIB accumulation. 5. 5. A model in which the carrier operated electro-neutrally, returning one K+ for each Na+ and AIB brought in, comes closer to satisfying the data from the studies of the steady state. However, there are a few discrepancies and the hypothesis does require a 100% efficiency of coupling between the ion fluxes and the uptake of AIB to explain most of the data from the steady-state studies. © 1969.
