As part of the research programme of the Atmospheric Environment Service, a large, sensitive weighing lysimeter was constructed on the Atmospheric Environment Service research site north of Toronto. Features of the instrument include: measurement of runoff, percolation, drainage of water held in tension, addition of water from below during dry conditions, equalization of soil temperature profile inside with that outside the lysimeter, humidity control to eliminate condensation, low area of discontinuity at the rim to reduce edge effects, and quasimonolith filling. This paper describes the construction and installation of the lysimeter, the problems encountered and some infrequently considered sources of error.