Estimates of Na+ pumping capacity were made using Na+-loaded canine iliac arteries. Ouabain-sensitive uptake of 204Tl or 86Rb was used to measure near-maximal pump rates and [3H]ouabain binding to measure the number of pump sites. Compared with Rb+, Tl+ had the higher affinity for the pump and showed better signal-to-noise characteristics. Maximal uptakes were 0.545 μmol∙g−1∙min−1 for Rb+ and 0.40 μmol∙g−1∙min−1 for Tl+ Specific ouabain binding (Kd: 28.62 ± 0.58 nM) was inhibited by external K+, Tl+, and Rb+ and a maximal binding of 51.6 pmol/g wet weight translated into 3.2 × 1013 sites per gram wet weight. Using these values, the maximal values of K+ transported per pump site per minute lie between 7752 and 10562. If each activation of the pump moves 2K+, the turnover rates could lie between 3876 and 5281 per minute.