At the present time, little is known concerning the electrophysiology of the cells of the macula densa and whether or not these cells are electrically responsive to alterations in luminal fluid composition. To investigate this issue, cortical thick ascending limbs (CTAL) containing macula densa and attached glomeruli were dissected from rabbit kidney and the CTAL perfused in vitro. Basolateral membrane potential (V(bl)) was measured with microelectrodes in macula densa cells and, for comparison, in cells of the CTAL. Macula densa V(bl) averaged -56.5 ± 7.6 mV (n = 4) at a perfusate NaCl concentration of 10 mM, -54.7 ± 3.1 mV (n = 22) at 20 mM NaCl, -35.6 ± 3.9 mV (n = 16) at 45 mM NaCl, and -25.5 ± 2.6 mV (n = 32) at 150 mM NaCl. Thus macula densa V(bl) depolarized markedly (31 mV) when luminal perfusate [NaCl] was increased from low to high values. In contrast, V(bl) measured in CTAL cells averaged -62 ± 6.1 mV (n = 6) in 45 mM NaCl and did not change significantly as perfusate NaCl was increased to 150 mM. In the presence of 150 mM NaCl, luminal application of furosemide (50 μM) produced a small (3.5 ± 1.1 mV, n = 16) but statistically significant (P < 0.02) hyperpolarization in macula densa cells, whereas CTAL cell V(bl) hyperpolarized markedly (20 ± 5.7 mV, n = 6) with addition of furosemide. Finally, neither macula densa cells nor the CTAL cells changed V(bl) when 45 mM NaCl solution was made hypotonic by removing mannitol. These electrophysiological measurements show clear differences between macula densa cells and CTAL cells and indicate that the macula densa cells are sensitive to changes in luminal [NaCl].