To examine the basolateral Cl transport mechanisms of proximal convoluted tubules (PCT), intracellular Cl activity (AiCl) was measured with double-barreled Cl-selective microelectrodes. When rabbit PCT were perfused in vitro with high Cl, low HCO3, and bathed with ultrafiltrate-like solutions, AiCl was 29.9 +/- 0.4 mM and basolateral membrane voltage (Vbl) was -47.7 +/- 0.4 mV (n = 247). Possible basolateral Cl transport mechanisms that we examined were as follows: Cl conductance, KCl cotransport, and Na-dependent Cl-HCO3 exchange. Cl conductance was negligible, since the voltage clamp of Vbl to 30 mV above and below the spontaneous Vbl did not change AiCl even in the absence of luminal Cl. KCl cotransport was suggested by 1) increasing bath K, increased AiCl, and 2) decreasing bath K decreased AiCl. KCl cotransport was Na independent and 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS), barium, and furosemide insensitive. Na-dependent Cl-HCO3 exchange was suggested by 1) bath HCO3 reduction increased AiCl, which was greatly inhibited by bath Na removal or bath SITS, and 2) bath Na removal increased AiCl, which was completely blocked by bath SITS. We conclude that 1) Cl conductance is negligibly small at the basolateral membrane and 2) SITS-insensitive KCl cotransport and SITS-sensitive Na-dependent Cl-HCO3 exchange are present at the basolateral membrane.