Effect of elevated pressure (up to 0,8 MPa) on NH3 removal from ground water in nitrification biofilter with mature filtration bed was studied under laboratory conditions. The NH3 N content decreased from 0.8 to 1.0 mg/L in raw water down to 0.1 mg/L after nitrification. The nitrification was influenced neither by elevated nor by oscillating pressure. The backwashing of biofilters contributed to an increase in their efficiency.