Results are reported from an investigation of the hydraulics of flow in experimental apparatus simulating nominally horizontal simple and branching drains of plumbing systems. The data are correlated with limited findings in an earlier, unpublished NBS study the results of which have been utilized in current plumbing codes. The need for further research is pointed out, particularly in relation to hydraulic performance of drain systems as affected by steep slopes, drain storage volume, energy losses at stack bases, attenuation of water depths and discharge rates in long drains, and large drain diameters.