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The use of electron spin resonance for measurement of the ground state population (GSP) of Cr3+in a ruby rod during optical pumping is described. For a 7.7 cm long rod at 90°K and a mirror reflectance product , the GSP at laser threshold was found to be 55 ± 5 percent in good agreement with theory and previous optical measurements. At room temperature, a limiting value of approximately 30 percent for the GSP was approached at high pump energies. Observations of the relative thresholds at 90°K as a function of mirror reflectivity indicated that the2E excited state absorption cross section at the laser wavelength is less than cm2. Detection of total internal reflection oscillations in cooled ruby lasers from ESR observations is also described.