Rat (r) OCT2 was identified as the second member of the organic cation transporter (OCT) family, and is predominantly expressed in the kidney. We reported previously that rOCT2 was responsible for the gender differences in renal basolateral membrane organic cation transport activity. As renal rOCT2 expression in males is much higher than that in females, we hypothesized that rOCT2 expression may be under the control of sex hormones. Treatment of male and female rats with testosterone significantly increased the expression levels of rOCT2 mRNA and protein in the kidney, whereas estradiol treatment moderately decreased the expression levels of rOCT2. There was no regulation of renal rOCT1 mRNA expression by testosterone or estradiol. Treatment of male and female rats with testosterone significantly stimulated the tetraethylammonium (TEA) accumulation by renal slices, whereas estradiol treatment caused a decrease in the TEA accumulation by slices from male but not female rats. The present findings suggested that testosterone up-regulates renal rOCT2 expression and estradiol moderately down-regulates rOCT2.