Transport of α-aminoisobutyric acid (AIB) was studied in microdissected pancreatic islets of obesehyperglycaemic mice. These islets are characterized by an unusually high proportion of insulin-secretingβ-cells. The following main observations were made. 1. The islet cells exhibited a concentrative uptake of AIB, yielding distribution ratios much higher than unity. The uptake was depressed by anoxia and by the omission of sodium from the medium. 2. L-Alanine and L-methionine, but not L-leucine, reduced the accumulation of AIB in islets which had been preincubated in the absence of exogenous amino acid. After preloading with L-alanine there was an enhanced uptake of AIB. 3. D-Glucose, D-galactose, diazoxide, dibutyryl cyclic 3′,5′-AMP, and D-mannoheptulose had minor or insignificant effects on the uptake of AIB. 4. Efflux of AIB from the islet cells was stimulated by exogenous AIB or L-alanine. It is concluded that the pancreaticβ-cells contain a mechanism for transport of neutral amino acids similar to the A-system of many other cells. The stimulating effect of glucose on insulin biosynthesis does not seem to be mediated by this system. Co-transport of amino acid and sodium is probably not enough to elicit a discharge of insulin from matureβ-cells.