Chronic renal failure (CRF) is accompanied by adaptive changes in electrolyte reabsorption in the thick ascending limb of Henle of surviving nephrons. To study the cellular mechanism of this adaptation, we measured intracellular cAMP in micro-dissected medullary thick ascending limb (mTAL) segments in rats with CRF. mTAL exhibited in CRF an increase of basal cAMP from 25.6 ± 10.0 in controls to 65.8 ± 11.3 fmol mm−1 tubule in CRF (P < 0.05). Vasopressin and calcitonin stimulated mTAL adenylate-cyclase in a dose-dependent manner in controls but failed to stimulate in CRF. Likewise, maximal stimulation with 10−3 M3-isobutyl-1-methylxanthine (IBMX) plus 10−5 Mforskolin increased cAMP in controls to 63.0 ± 16.0 but not in CRF, where maximal stimulated values remained at 63.1 ± 18.8 fmol mm−1 tubule (P NS). Alpha2-adrenoreceptor activation with clonidine at concentrations ranging from 10−8 to 10−6 Mdiminished cAMP production by 37% in CRF (P < 0.05), whereas no differences were found in controls. Thus, the basal intracellular cAMP is increased in rat mTAL in CRF. The finding that neither forskolin nor vasopressin were able to further augment intracellular cAMP would suggest that stimulatory pathways of the adenylate-cyclase system are activated in the basal state. However, mTAL cells in CRF seem to retain the response of normal epithelium to inhibitory pathways such as the one mediated by alpha2-adrenoreceptors.