Nitrogen-16 is produced in the coolant of water-cooled reactors from the 16O(n,p)16N reaction, and its rate of generation increases with an increase in reactor power level. Therefore, measurement of gamma rays emitted by 16N in the primary coolant can be used to monitor the reactor power level. Measurements have been made with a locally fabricated argon-filled three-electrode ionization chamber. The (I, V) characteristics of the chamber filled with argon have been studied with 16N gamma rays at the Pakistan Research Reactor-1 (PARR-1). The ionization current was measured at 1-, 3-, 5-, 7-, and 10-MW power levels. The measured ionization current was found to increase linearly with the power level. The plateau region of the chamber was observed to start at an applied voltage of 400 V, and the chamber operating voltage was found to be 600 V at an argon gas pressure 1.38 MPa. An empirical relation between reactor power and ionization current was developed. The (1/I, 1/V) and (I, 1/V2) curves elucidate the initial and volume recombination losses, respectively. The volume recombination losses were found to be relatively smaller than the initial recombination losses. These losses were found to increase with increasing power level. However, the increase in the initial recombination losses was slightly greater than the volume recombination losses.