Angiotensin II (ANG II) is a modulator of tubuloglomerular feedback, yet the site of its action remains unknown. Macula densa (MD) cells sense changes in luminal [NaCl] ([NaCl]_L) via a Na:2Cl:K cotransporter and these cells do possess ANG II receptors. We tested whether ANG II regulates Na:2Cl:K cotransport in MD cells. MD cell Na⁺ concentration ([Na⁺]_i) was measured using SBFI with fluorescence microscopy. Resting [Na⁺]_i in MD cells was 27.7 ± 1.05 mM (N = 138) and increased ( Na⁺_i) by 18.5 ± 1.14 mM (N = 17) at an initial rate (Na⁺_i / t) of 5.54 ± 0.53 X10^-4 U/s upon increasing [NaCl]_L from 25 to 150 mM. Both Na⁺_i and Na⁺_i / t were inhibited by 80% with 100 µM luminal furosemide. ANG II (10^-9 or 10^-12 M), added to the lumen, increased MD resting [Na⁺]_i and [NaCl]_L-dependent Na⁺_i and caused a 2-fold increase in Na⁺_i / t. Bath (10^-9 M) ANG II also stimulated cotransport activity and there was no additive effect of simultaneous addition of ANG II to hath and lumen. The effects of luminal ANG II were furosemide-sensitive and abolished by the AT1 receptor blocker, candesartan. ANG II at 10^-6 M failed to stimulate the cotransporter, while increased cotransport activity could be restored by blocking AT² receptors with PD 123,319. Thus, ANG II may modulate TGF responses via alterations in MD Na:2Cl:K cotransport activity.