Previous investigations have shown that the masking capacity of a prolonged auditory stimulus remains constant over its duration in spite of its great diminution in loudness. The experiments which follow were designed to determine the relation between the masked threshold and the duration of the masked stimulus. The results show that (1) within limits, as the duration of a pure tone is increased less noise is required to mask the tone, (2) the magnitude of this effect increases as a function of frequency of the masked tone but appears to be independent of its SPL, and (3) in general, these effects diminish somewhat with practice.