Sodium-dependent citrate uptake (v) by rabbit renal brush border vesicles was studied as a function of pH and Ca2+, citrate, and succinate concentrations to evaluate the hypothesis that factors which alter renal citrate reabsorption also alter v. At pH 8 a 10-fold increment in citrate concentration (up to 3.3 mM) was required to achieve the same v observed at pH 7. This provides proof that the protonated citrates are the effective substrates for v. At higher citrate concentrations (5-10 mM) and pH 8, substrate inhibition of v occurred. Studies of the substrate concentration dependency of v at various pH levels and calcium concentrations indicated that v was competitively inhibited by citrate3-. Changes in calcium concentrations altered v by altering the protonated citrate and citrate3- concentrations; v showed countertransport with succinate. Sodium-dependent succinate uptake (vs) was not pH dependent but was more inhibited by 1 mM citrate at pH 7 than at pH 8, indicating that the protonated citrates are inhibitors of vs. However, citrate3- was also found to inhibit vs. Thus, changes in v at the brush border membrane due to the effects of pH or calcium concentration on the protonated citrate concentration and to competition between polycarboxylates may be relevant to changes in renal citrate reabsorption in vivo.