The behaviour of rat kidney cortex phosphoenolpyruvate carboxykinase has been investigated under conditions of triamcinolone administration and ammonium chloride acidosis. The concentration of phosphoenolpyruvate carboxykinase as measured by enzyme activity and immunotitration was elevated under both conditions. The mechanism of induction is different in the two cases. At doses which produce maximum stimulation, the effects of steroid and ammonium chloride were additive; only the increment in enzyme activity produced by steroid was blocked by actinomycin D. PHOSPHOENOLPYRUVATE CARBOXYKINASE ACTIVITIES IN ALL CONDITIONS INVESTIGATED SHOW SIMILAR BEHAVIOR IN DILUTE EXTRACTS: these experiments involved antibody titration, stability studies, and molecular weight determinations on sucrose gradients. The molecular weight of phosphoenolpyruvate carboxykinase was also studied in undiluted extracts prepared by high-speed centrifugation; values were determined from sedimentation data obtained with a moving-partition cell as described by Yphantis and Waugh. Under these conditions, the apparent molecular weight of phosphoenolpyruvate carboxykinase was increased from 83,000 to 128,000 by ammonium chloride acidosis. These results are discussed and a hypothesis regarding the mechanism of phosphoenolpyruvate carboxykinase regulation in kidney cortex is presented.