The effect of masking noise on the temporal summation of loudness is investigated here by performing loudness balances between a standard 500-msec tone burst (1000 Hz at either 20-, 50-, or 80-dB SPL) and either masked or unmasked comparison tone bursts (1000 Hz with durations between 10 and 640 msec). In all but two instances, the obtained functions relating SPL for equal loudness to stimulus duration could be plotted as two line segments that met to form a knee. The slopes of the line segments at durations less than the kneepoints are altered by the masking noise, becoming less steep with increased masking. The rate of the slope decrease is related to the standard sound pressure level (SPL) and is greatest using the 80-dB SPL standard and least with the 20-dB SPL standard. Temporal summation of loudness continues at durations above the kneepoints. However, the obtained slopes are less than those found below the knee, and are independent of the test conditions. The slope changes are found to be related to the noise producing a power transformation on the operating characteristics of the auditory system. These latter findings are discussed in relation to Zwislocki’s quantitative theory of the temporal summation of loudness.