Sildenafil citrate, a phosphodiesterase-5 (PDE5) inhibitor widely used for the treatment of erectile dysfunction was investigated for its interaction with the zinc-enzyme carbonic anhydrase (CA, EC 4.2.1.1), as it has in its molecule a piperazine moiety also found in some CA activators (CAAs). Sildenafil was a potent, low micromolar activator of several CA isozymes, such as CA I, VA and VI (K(A)s in the range of 1.08-6.54microM), and activated slightly less the isoforms CA III, IV and VA (K(A)s of 13.4-16.8microM). CA isozymes II, IX, XIII and XIV showed activation constants in the range of 27.5-34.0microM, whereas the least activated isoforms were CA VII and XII (K(A)s of 72.9-73.0microM). Sildenafil citrate was also given orally to Sprague-Dawley rats at 1mg/kg body weight. Red blood cell CA activity was inhibited in the treated animals at 3-5h post-administration (in the range of 60-85%), probably due to NO/nitrite formed by PDE5 inhibition or by another, unknown mechanism. Whether CA activation by sildenafil has clinical consequences in humans is beyond the scope of the present work and warrants further studies.