Perfusion of the renal medulla and osmotic hypertonicity of its interstitium are the two important features of this zone which can influence body fluid homeostasis, especially arterial blood pressure. Separate manipulation of the two variables is best obtained with the intramedullary infusion of active agents. In this study, a set-up combining the possibility of infusion into the medulla with measurement of local blood flow (MBF, laser-Doppler flux) and extracellular ion concentration (tissue electrical admittance, Y) was used to determine effects of intramedullary indomethacin (Indo) and adenosine (Ado) in anaesthetized rats. Intramedullary Indo, 1 mg kg<sup>–1 </sup>h<sup>–1</sup>, significantly increased tissue Y, by 12 ± 3%, and significantly decreased MBF by 20 ± 3%.There was also an unexplained increase of sodium excretion (U<sub>Na</sub>V) by 169 ± 24% and of urine flow (V) by 62 ± 6% (n = 10, both p < 0.03). Intramedullary Ado, 5 µg kg<sup>–1</sup> h<sup>–1</sup>, did not alter Y, MBF or U<sub>Na</sub>V, whereas V increased 45 ± 6% and urine osmolality decreased 25 ± 4% (both changes significant). Elevation of medullary interstitial Ado to a level that did not alter MBF or U<sub>Na</sub>V induced a moderate defect of urine concentration that was not due to a decrease in ionic medullary hypertonicity.