The loudness of a steady auditory stimulus declines during the first few minutes of the application of that stimulus. This decrease in loudness is so great that it is important to know whether or not the masked threshold also changes during the initial period of stimulation. In the present investigation the degree of perstimulatory fatigue and the amount of masking were both determined for the same stimulus. It is shown that the masked threshold is essentially independent of the duration of the masking stimulus.