Effects of Na and K ions on the active Na transport in guinea-pig auricles. 
1.  The effect of Na and K ions on active Na transport was studied in guinea-pig auricles by means of flame photometry.  2.  The Na influx into preparations rewarmed in Tyrode's solution after cooling was estimated to be about 1.05 mmole/l fibre water - min (l.f.w.-min) or c. 8 pmole/cm2 - s. Intracellular Na ions enhanced the active Na efflux over a wide range of concentrations.  A decrease in the extracellular Na concentration ([Na]o) had no major effect on the active Na efflux.  3.  Extracellular K ions initiated an active Na efflux from rewarmed auricles with an elevated [Na[i over a narrow range of K concentrations ([K]o).  4.  Assuming Michaelis-Menten kinetics the maximal active Na efflux activated by internal Na ions was calculated to be about 4 mmole/l.f.w. -  min (30 pmole/cm2 - s).  Half maximal Na efflux occurred at about 22 mmole/l.f.w. [ Na]i.  The maximal K-activated active - min (28 pmole/cm2 - s) and was half maximal at a [K]o of about 0.2 mM. 5.  It is tentatively concluded that the maximal active Na efflux from guinea-pig atria is 3--4 times larger than the physiological flux.  Under normal conditions active Na efflux in heart is mainly regulated by variations of [Na]i.