In most cell types, a significant part of cationic amino acid transport is mediated by the so-called system y⁺, characterized by selectivity for cationic amino acids, half maximal activity at cationic amino acid concentrations (K_M) of 0.1 to 0.2 mM, Na⁺ and pH independence, and strong stimulation of transport by substrate at the trans-side of the membrane (trans-stimulation). Protein kinase C (PKC) activation has been reported both to stimulate and to down-regulate system y⁺ activity, suggesting a differential action of PKC on different carrier proteins mediating system y⁺ activity. Three such carrier proteins, that belong to the family of cationic amino acid transporters (CATs) have been identified. They underlie a complex regulation at the transcriptional, posttranscriptional and activity level. Recent evidence indicates that individual CAT isoforms are necessary for providing substrate for nitric oxide synthesis, e.g. CAT-1 for Ca²⁺-independent NO production in endothelial cells and CAT2B for sustained NO production in macrophages. The function and regulation of the CAT proteins will be reviewed with emphasis on the regulation of these transporters by PKC.