The movement and transformation of nitrogen under seven septic tank installations was studied. Samples of the effluent and of the soil solution beneath the discharge systems were taken and analysed for ammonium, nitrate and pH. Most of the nitrogen came from household toilets, and after passage through a septic tank, the nitrogen was mostly in the ammonium form. It remained in this form while it was in the ponded effluent above the slime layer in the absorption systems. Once it had passed through the slime layer into the unsaturated, aerobic soil below, it was usually oxidised to nitrate within 0-5 m. This was accompanied by a drop in pH of about two units. However, in one case in which the distance between the top of the slime layer and the water table was less than O'8 m, ammonium was not oxidized to nitrate. This was taken to indicate that the saturated zone extending below the absorption system overlapped the capillary fringe and consequently the soil was anaerobic. Accumulation of organic nitrogen was restricted to the 0-5 m slime layer which accumulated on the sandy base of the soak wells and leach drains. The low cation exchange capacity of the soil and the high rate of conversion of ammonium in the effluent to nitrate in the unsaturated soil resulted in almost all of the nitrogen from the septic tanks entering the groundwater except that lost to plant uptake.