The juxtaglomerular apparatus fulfils several important regulatory functions in the kidney, such as tubuloglomerular feedback (TGF) control and control of renin release. The macula densa (MD) cells sense the fluid load by perceiving the distal NaCl concentration via a Na-K-2Cl cotransport system in the luminal cell membrane. It has been proposed that macula densa cell activation may involve changes in intracellular cytosolic free calcium concentration ([Ca2+]i), as one link in the chain of events activating TGF or releasing renin. We therefore investigated the changes in the intracellular calcium concentrations with fura-2, using a video system, in macula densa cells, and compared them with the changes in the corresponding concentrations in the ascending limb of the loop of Henle (c-TAL). The results show that our technique for analysing intracellular cytosolic free calcium in isolated perfused tubules is valid for this purpose, and the Kd value obtained was similar to that found by Grynkiewicz et al. (1985). The intracellular cytosolic free calcium concentration was about 90 nM both in the macula densa and c-TAL cells, and the macula densa cell intracellular cytosolic free calcium concentration increased by about 20 nM when the tubular lumen was perfused with Na and Cl at low concentrations. No significant changes were noted when furosemide was added to the perfusion solutions. We consider it hardly likely that this small change in intracellular cytosolic free calcium concentration can be entirely responsible for full activation of renin release or full inactivation of the TGF control mechanism. It would seem that the signal transmission from the macula densa cells could occur by other routes than through activation of intracellular cytosolic free calcium concentration.