Ion channels have been assigned a pivotal importance in various sperm functions and are therefore promising targets for contraceptive development. The lack of data on channel functionality and pharmacology has hampered this goal. This is a consequence of technical problems of applying electrophysiological techniques to spermatozoa due to their small size and form. By using a laminin coating to increase adherence of spermatozoa and nystatin in the patch pipette for pore formation, we have adapted the whole-cell recording technique to study currents in mature uncapacitated bovine spermatozoa. Employing these conditions, in the head region, patched spermatozoa could be transferred into the whole-cell configuration. For the first time we document an outward rectifying current in mature bovine spermatozoa was blocked by tetraethyl ammonium (TEA) chloride. The observation of a shift in the reversal potential as a response to changes in the extracellular concentration of K+ ions allowed us to identify this current as K+ selective. This result shows that K+ channels in the head region of mature uncapacitated bovine spermatozoa can be suitably investigated using the whole-cell recording patch-clamp technique.This work was supported by Fondef - Conicyt ( No D05I10416) projects ( Chile). HU acknowledges funding from FAPESP (Fundacao de Amparo a Pesquisa do Estado de Sao Paulo, project No. 2006/61285-9) and CNPq (Conselho Nacional de Desenvolvimento Cientifico e Tecnologico), Brazil. The authors wish to acknowledge Dr. Paulo Boschcov for his help in preparing the manuscript and Dr. Armanda Gameiro, Department of Chemistry, Binghampton University, NY, for comments on the manuscript.5FONDEFhenning@iq.usp.br; fromero@ufro.clFondef - Conicyt [D05I10416]; FAPESP (Fundacao de Amparo a Pesquisa do Estado de Sao Paulo) [2006/61285-9]; CNPq (Conselho Nacional de Desenvolvimento Cientifico e Tecnologico), Brazil6FONDEF54