The interactions of sulfonylureas and a novel anti-diabetic drug, nateglinide, with rat renal organic anion transporter (rOAT1) expressed in Xenopus laevis oocytes were studied. Uptake of p-aminohippurate via rOAT1 was markedly inhibited by glibenclamide and nateglinide, and moderately by chlorpropamide and tolbutamide. The inhibition constant values ( K i) for chlorpropamide, glibenclamide, tolbutamide and nateglinide were 39.5, 1.6, 55.5 and 9.2 μM, respectively. Kinetic analysis showed that the inhibition of p-aminohippurate uptake by glibenclamide was competitive. Sulfonylureas examined and nateglinide did not show a trans-stimulation effect on [ 14C] p-aminohippurate efflux from rOAT1-expressing oocytes. There was no stimulation of [ 3H]glibenclamide uptake via rOAT1. These findings suggested that sulfonylureas and nateglinide interact with rOAT1, but these drugs are not translocated via the transporter.