These studies were conducted to determine if the prostaglandin-synthesis inhibitor indomethacin or the protein kinase C (PKC) inhibitor staurosporine affect the inhibition of osmotic water permeability (P(f)) by the alpha-2 (α2) agonist dexmedetomidine in the rat inner medullary collecting duct (IMCD). Terminal IMCDs from Wistar rats were perfused and P(f) was increased with either 220 pM arginine vasopressin (AVP) or 0.1 mM 8-chlorophenylthio cyclic adenosine monophosphate (8CPTcAMP). All agents were added to the bathing solution. Dexmedetomidine at 100 nM inhibited both AVP- and 8CPTcAMP- stimulated P(f). When P(f) was increased by AVP, indomethacin at 0.1 mM or 5 μM reversed the dexmedetomidine-induced inhibition by 68% and 43%. respectively. When P(f) was increased by 8CPTcAMP, indomethacin at 0.1 mu or 5 μM reversed inhibition by 83% and 70%, respectively. Indomethacin increased AVP and 8CPTcAMP-stimulated P(f) by 20 to 30% and dexmedetomidine inhibited the AVP+ indomethacin-stimulated P(f). Staurosporine at 10 nM yielded similar results. Results suggest that PKC and prostaglandins are involved in the α2 mediated mechanism, and staurosporine and indomethacin- sensitive cellular mediators modulate basal P(f).