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Concentrations of hydrogen sulfide (H2S), ammonia (NH3) and carbon dioxide (CO2) were determined in 12 commercial swine barns with slatted floors and sub-floor slurry storage pits, prior to, during and immediately after mixing of slurry, on 19 different days over a two-year period. The objective was to establish the extent to which hazardous conditions could develop due to manure gases during slurry mixing, and if specific manure properties or composition constituents could be related to the observed gas release. Only H2S was determined continuously at different locations in the barns and in manure pit exhaust air, using electrochemical cells with continuous data loggers. Time-weighted average and instantaneous concentrations of all three gases were determined in the barns using Draeger long-term, short-term and diffusion detector tubes. Under normal ventilation, concentrations of H2S, NH3, CH4 and CO2 were at acceptable levels when the slurry was undisturbed. Immediate and very rapid release of H2S at high concentrations (but not other gases) at the start of mixing of slurry indicated a potential hazard for the operator and animals. These concentrations decreased as mixing progressed. Time-weighted concentration measurements did not always indicate this rapid concentration increase, which reached a lethal level in one case. Use of well-functioning pit exhaust fans and avoiding splashing and free falling of slurry in the pit reduced H2S concentration above the slats. The most important factor in H2S release seemed to be slurry mixing and air movement in the barn rather than the composition of the manure or other factors.