1. In order to determine the stoichiometry of K+ uptake and ATP consumption by Na+, K(+)-ATPase in the isolated, perfused mandibular gland of the rat, oxygen consumption and net K+ uptake from the vascular side were measured during the recovery period following K+ depletion by stimulation with acetylcholine in combination with ouabain. 2. Acetylcholine (10(-6) M) induced fluid secretion and an initial, transient release of K+ from the gland. Addition of ouabain suppressed salivary fluid secretion and caused an additional, dose-dependent, transient release of K+. 3. With acetylcholine (10(-6) M), the oxygen consumption of the gland increased to 62.4 +/- 4.2 microliters/(g min) from a resting value of 12.9 +/- 1.2 microliters/(g min). The increased oxygen consumption was suppressed by ouabain, in a dose-dependent fashion. 4. Withdrawal of acetylcholine and ouabain induced a net uptake of K+ and, simultaneously, an increase in oxygen consumption. The cumulative K+ uptake and the increment of oxygen consumption during the recovery period were dependent on the concentration of used ouabain. 5. The rates of K+ uptake and ATP hydrolysis were compared during recovery, assuming that six moles of ATP were hydrolysed for each mole of oxygen consumed. The data obtained during the later phase of the recovery lay on a single straight line with a slope of 1.8 for each of the various concentrations of ouabain, suggesting that the relationship between K+ uptake and energy consumption was linear. 6. Assuming the K+:ATP stoichiometry of the Na+, K(+)-ATPase to be 2:1, K+ would appear to be transported with ca 90% efficiency by Na+, K(+)-ATPase in the rat mandibular gland. Using 1.8-2.0 as a K+/ATP stoichiometry, the rate of primary active K+ uptake and the corresponding passive K+ efflux during secretion were estimated to be 20-22 mumol/(g min) from the oxygen consumption and the net K+ flux. 7. The passive K+ efflux was estimated, from the initial rate of K+ release caused by addition of 10(-3) M-ouabain, to be 30 mumol/g min). The discrepancy between the estimated active K+ uptake and passive K+ release (8-10 mumol/(g min] could be attributed to a secondary active K+ uptake process such as Na(+)-K(+)-2Cl-symport.