The uptake of beta-hydroxy-beta-methyl-glutarate (HMG) and beta-hydroxy-butyrate (beta-HB) by renal brushborder membrane vesicles prepared from normal and starved rats was examined. HMG and beta-HB uptake show a Na+ gradient-induced overshoot, suggesting luminal cotransport of these organic acids. Kinetic analysis of HMG and beta-HB uptake revealed a single component carrier system and a diffusional component for each compound. Vesicles from starved rats exhibit the same transport characteristics as those from normal rats. The transport interactions of other organic acids with HMG were examined and revealed that citrate is a competitive inhibitor, which implies that the compounds share a common organic acid carrier.