<jats:p>A study is presented of the effect of Zn<jats:sup>2+</jats:sup> on the enzymatic properties of the bovine‐heart cytochrome‐<jats:italic>bc</jats:italic><jats:sub>1</jats:sub> complex. Micromolar concentrations of Zn<jats:sup>2+</jats:sup> reversibly inhibit the cytochrome‐<jats:italic>c</jats:italic> reductase activity of either the cholate‐solubilized or liposome‐reconstituted complex. Kinetic analysis of the redox reactions of the cytochromes indicate that Zn<jats:sup>2+</jats:sup> affects the activity of the complex at the quinol oxidation site. The following have been determined: (a) Zn<jats:sup>2+</jats:sup> inhibits the pre‐steady‐state reduction of cytochrome <jats:italic>c</jats:italic><jats:sub>1</jats:sub> by duroquinol either in the absence or in the presence of antimycin, (b) it does not inhibit the reduction of <jats:italic>b</jats:italic> cytochromes in the absence of antimycin or in the presence of myxothiazol, (c) it inhibits cytochrome‐<jats:italic>b</jats:italic> reduction in the presence of antimycin. Furthermore Zn<jats:sup>2+</jats:sup> inhibits the antimycin‐promoted oxidant‐induced extrareduction of <jats:italic>b</jats:italic> cytochromes.</jats:p><jats:p>Addition of Zn<jats:sup>2+</jats:sup> to reduced <jats:italic>bc</jats:italic><jats:sub>1</jats:sub> complex causes a red shift in the absorption spectrum of cytochrome <jats:italic>b</jats:italic><jats:sub>566</jats:sub> and a substantial decrease in the signal intensity of the EPR spectrum of the Fe‐S protein. This is interpreted as an interaction of Zn<jats:sup>2+</jats:sup> with the 2Fe–2S‐cluster region of the Fe‐S protein, thus giving rise to inhibition of the reductase activity and of the antimycin‐insensitive reduction route of <jats:italic>b</jats:italic> cytochromes.</jats:p><jats:p>A Scatchard‐plot of <jats:sup>65</jats:sup>Zn<jats:sup>2+</jats:sup> binding to the native isolated complex gave a straight line from which a value of three binding sites and a single dissociation constant of 3 × 10<jats:sup>−6</jats:sup> M can be calculated, which is practically equal to the concentration causing 50% inhibition of electron flow.</jats:p>