An apparatus for diluting a sample with a diluent, comprising a fixed volume measuring chamber, a fixed volume receiving chamber and fluid receiving relationship to the measuring chamber, a gas vent in the receiving chamber, a stop flow junction between the measuring chamber and the receiving chamber, a sample application site in fluid donating relationship to the measuring chamber, wherein the vertical height difference between the sample application site and the stop flow junction is insufficient to provide a hydrostatic pressure capable of overcoming backpressure at the stop flow junction when sample is applied to the sample application site, and the diluent application site and fluid donating relationship to the measuring chamber. Sample and diluent are added sequentially to the apparatus of the invention without requiring any intervening operations. Various means are provided for restarting flow, which causes diluent to wash the sample into the receiving chamber, where the two can be mixed.