Aquaporin-1 (AQP1) is a member of the membrane intrinsic protein (MIP) gene family and is known to provide pathways for water flux across cell membranes. We show here that cloned human AQP1 not only mediates water flux but also serves as a cGMP-gated ion channel. Two-electrode voltage-clamp analyses showed consistent activation of an ionic conductance in wild-type AQP1-expressing oocytes after the direct injection of cGMP (50 nl of 100 mM). Current activation was not observed in control (water-injected) oocytes or in AQP5-expressing oocytes with osmotic water permeabilities equivalent to those seen with AQP1. Patch-clamp recordings revealed large conductance channels (150 pS in K(+) saline) in excised patches from AQP1-expressing oocytes after the application of cGMP to the internal side. Amino acid sequence alignments between AQP1 and sensory cyclic-nucleotide-gated channels showed similarities between the cyclic-nucleotide-gated binding domain and the AQP1 carboxyl terminus that were not present in AQP5. Competitive radioligand-binding assays with [(3)H]cGMP demonstrated specific binding (K(D) = 0.2 microM) in AQP1-expressing Sf9 cells but not in controls. These results indicate that AQP1 channels have the capacity to participate in ionic signaling after the activation of cGMP second-messenger pathways.