The key-peck response of five pigeons was reinforced on a schedule whenever the interval between pecks at two response keys was between 1.0 and 2.33 seconds in the presence of a 2,500-Hertz tone or between 4.66 and 6.0 seconds in the presence of a 1250-Hertz tone. There was no tendency for responses of intermediate duration to occur when test tones of intermediate frequency were presented. This result clarifies a previous finding using a similar procedure but with a visual intensity stimulus dimension.