Uphill base-line uptake of a weak organic anion, fluorescein, in rat renal proximal tubules is stimulated by a transaminase inhibitor, aminooxyacetate (AOA, 1 mM). Considering that an inhibition of cytoplasmic aspartate aminotransferase can be accompanied by an elevation in the cytoplasmic alpha-ketoglutarate (KG) level, it was hypothesized that the effect of AOA on the fluorescein uptake was mediated through an augmentation of the KG gradient across the basolateral membrane, which was maintained owing to the Li-inhibitable KG reuptake in the cells. In order to test this hypothesis, an influence of Li on the stimulatory effects of AOA and KG on the fluorescein uptake was investigated. The hypothesis was supported by results showing that LiCl (5 mM) completely abolished the stimulatory effect of KG and attenuated almost 3 times that of AOA, whereas Li did not affect the base-line fluorescein uptake. Moreover, the stimulatory effects of AOA and KG on the uptake were of the same degree but failed to be additive, suggesting that they share a common mechanism. Modulatory influences of metabolic inhibitors, such as fluoroacetate, malonate, phenylpyruvate and D-malate, as well as of L-lysine on the effects of AOA and KG also were investigated to outline metabolic processes involved in these effects. It is concluded that KG metabolism related to its participation in the malate-aspartate shuttle, but not to its oxidation in the tricarboxylic acid cycle, is of importance for the AOA effect.