When oil is produced by water injection, sulphide formation (souring) can be stimulated. Souring is often caused by sulphate-reducing bacteria (SRB) which oxidize organic carbon (oil organics) with sulphate in the injection water to CO(2) and sulphide. As a result, H(2)S concentrations in the produced water, oil and gas gradually increase. A consequence can be that the piping infrastructure must be redesigned from sweet to sour service. A relatively novel biotechnology aimed to remedy souring is to add nitrate to the injection water. Nitrate tracks the injection water effectively and its cost allows continuous and field-wide treatment. In a field-wide nitrate injection, the injection water (approximately 3,500 m(3)/day) was amended continuously with 2.4 mM (150 ppm) nitrate. Three points in the injection water system and 12 production wells were monitored by sampling every 2 to 3 weeks. The concentrations of sulphide, sulphate, nitrate, nitrite and ammonia in injection and produced waters were determined as well as the activities of nitrate-reducing bacteria (NRB). Field-wide nitrate injection gave a 70% drop of aqueous sulphide within the first 5 weeks, after which the concentration recovered somewhat for the next 20 weeks. The activity of NRB increased thoughout this period, indicating the possibility of further decreases in souring in the future.