Membrane inlet mass spectrometry (MIMS) was used to investigate denitrification by Pseudomonas stutzeri in a static lake water column. Continuous real-time measurement of gases enabled the dynamics of the process to be investigated. Concentrations of 17 mmol l(-1) nitrate and 10 mmol l(-1) nitrite were identified as optimal for denitrification under nutrient-limited conditions (i.e., produced the highest concentrations of N-2) Available carbon was the major rate-limiting factor in lake water when nitrate or nitrite was present. No stratification of the process with depth was observed, and aerobic denitrification was apparent under all the conditions employed. The rate of denitrification was dependent on cell concentration, and possible limitations of the usefulness of MIMS under environmentally modelled conditions were identified for environments containing low numbers of bacteria.