(A) Original current traces from whole-cell voltage-clamp recordings of noncapacitated human spermatozoa. Inward- and outward currents were elicited with voltage ramps as depicted. Pipette solution was: 140 mM NMDG, 100 mM Hepes, 5 mM EGTA, 5 mM EDTA, 330 mOsmol, pH 7.3, composition of bath solution was: 500 nM progesterone, 100 mM Hepes, 130 mM NMDG, plus X mM Ca2+, Ba2+ or Mg2+ as depicted, 317 mOsmol, pH 7.4. When the major permeable extracellular cation was Ca2+ or Ba2+, negative membrane potentials induced concentration-dependent inward currents. In the presence of Mg2+, CatSper currents remained at baseline level (0 mM), indicating that human CatSper is not permeable for Mg2+. (B) Quantification of current densities (pA/pF) for either Ca2+, Ba2+ or Mg2+ inward currents through CatSper.