1. The O2 consumption and lactic acid production of the guinea‐pig's taenia coli have been studied in relation to the active Na‐K transport, in order to estimate the ratio: active Na extrusion/active K uptake/ATP hydrolysis. 2. By applying different procedures of partial metabolic ingibition, it was found that a reactivation of the active Na‐K transport in K‐depleted tissues could occur in an anaerobic medium, provided glucose was present and in an aerobic medium free of added metabolizable substrate. The active Na‐K transport was rapidly blocked in an anaerobic‐substrate free medium. 3. Readmission of K to K‐depleted tissues under aerobic conditions stimulates both O2 consumption and lactic acid production. While the O2 consumption creeps up slowly and requires 50 min to reach control values, the aerobic lactic acid production increases to a maximum within 10 min and decreases again during the next 50 min to its steady‐state value. 4. A reactivation of the Na‐pump in K‐depleted cells in a N2‐glucose medium causes an immediate increase of the lactic acid production, which decreases to its control value after 60 min. The maximal increase in anaerobic lactic acid production during reactivation of the Na‐K pump is a function of [K]O. The system can be cescribed with first order kinetics having a Vmax = 0–72 mumole.g‐1 f. wt. min‐1 and a Km = 1‐1 mM. 5. By varying the glucose concentration of [K]O during reactivation of the Na‐K pump, different Na‐K pumping rates can be obtained. The ratios net Na extrusion/ATP or net K accumulation/ATP amount to −1–32 +/− 0–19 (36) and 1‐02 +/− 0–11 (36), in the experiments with different glucose concentrations. Taking into account the interference by net passive fluxes, one can estimate a ratio:active Na transport/active K transport/ATP, of 1‐7/0–8/1. This ratio is not very different from the values observed in other tissues.