In 2003, 19 water providers and other agencies in North America sponsored the Maximum Performance (MaP) Testing of Popular Toilet Models, an element of which included the measurement of flush volumes for original and after-market flappers on over 50 different toilet models. That element is the subject of this paper. A listing of sponsors of the program is provided in the paper, along with a discussion of three specific tests involving flappers and flush volumes that included these methods: measuring the "out-of-the-box" flush volume with the fixture assembled in accordance with the manufacturer's instructions as provided in the manufacturer's packaging and with the OEM (original equipment manufacturer) flapper installed; the OEM flapper was replaced, where possible, with a commonly available standard aftermarket replacement flapper and the resulting flush volume was then measured and recorded; and, the OEM flapper was replaced, where possible, with each one of the three different types of adjustable flappers to determine the appropriate setting (dial or insert) to maintain 1.6- gpf/6.0-lpf. Includes tables.