SLC16A12 is a recently identified creatine transporter of unknown physiological function. A heterozygous mutation in the human SLC16A12 gene causes juvenile cataracts and reduced plasma guanidinoacetate (GAA) levels with an increased fractional urinary excretion of GAA. Our study with transgenic SLC16A12-deficient rats reveals that SLC16A12 is critical for tubular reabsorption of creatine and GAA in the kidney. Our data furthermore indicate a dominant-negative mechanism underlying the phenotype of humans affected by the heterozygous SLC16A12 mutation.