Aquaporin-1 (AQP1) inhibitors are predicted to have multiple clinical applications. Hg(++) is a non-specific and toxic AQP1 blocker. We compared compounds with reported AQP1 inhibition activity, including DMSO, Au(+++), Ag(+), tetraethylammonium and acetazolamide. Water permeability was measured by stopped-flow light scattering in erythrocytes and volume marker dilution in epithelial cells. Au(+++) inhibited AQP1 with IC50 approximately 14 microM, similar to 10 microM for Hg(++). DMSO slowed osmotic equilibration; however, the apparent inhibition was due to 'osmotic clamp' rather than AQP1 inhibition. Neither tetraethylammonium nor acetazolamide (to 10 mM) inhibited AQP1. Our data indicate the need to identify new AQP1 inhibitors.