[en] Increasing stimulation frequency and monensin were utilized in attempts to enhance Na⁺ influx and thereby elicit the maximum rate of ouabain-sensitive ⁸⁶Rb⁺ uptake in atrial muscle isolated from rat heart. Preparations were bathed in a K⁺-free, Rb⁺-containing Krebs-Henseleit buffer at 37⁰C. Increasing stimulation frequency between 0 and 9 Hz enhanced the rate of ouabain-sensitive ⁸⁶Rb⁺ uptake, whereas the uptake rate appeared to decrease at higher frequencies. Monensin (1.0 to 5.0 μM) increased the rate of uptake in atrial preparations stimulated at 6 Hz. A higher concentration of monensin produced a significant decline in uptake rate. The maximum rate of ouabain-sensitive ⁸⁶Rb⁺ uptake produced by monensin in preparations stimulated at 6 Hz was not significantly different from the maximum elicited by stimulation frequency alone. The results suggest that either method of increasing Na⁺ influx may be used to maximize the rate of ouabain-sensitive ⁸⁶Rb⁺ uptake and thereby estimate the Na⁺-pump capacity of rat atrial muscle