Many mechanically ventilated pig-finishing barns in the mid-western United States store manure in 2.5 m deep concrete pits directly below a fully slatted floor. Gas emissions from the animals, manure and urine on the slatted floor and other surfaces and manure in the deep pit impact indoor gas concentrations and ventilation emissions through wall and pit exhaust fans. Previous studies reported measured ammonia and hydrogen sulfide concentrations and emissions through wall and pit fans for a mechanically ventilated pig-finishing barn in Minnesota. The purpose of this study was to develop and evaluate a three-zone model describing ventilating air exchange, gas concentrations in two animal zones and the pit, and air exchange between the deep pit and animal zones. The model also included short-circuiting of inlet air. Ammonia, hydrogen sulfide and ventilation data from a field study supplied model inputs. Eighteen data sets were used; two gases (ammonia and hydrogen sulfide), three seasons (i. e. spring, summer and fall) and three pit ventilating rate treatments (i.e. 1, 2 and 4 pit fans). The model generated a range of unique acceptable solutions for 16 of the 18 combinations assessed. Model results indicated that the animal zone gas concentrations nearest the exhaust fans were always greater than the wall exhaust concentrations whenever there was inlet air short-circuiting. Results indicated that inlet air short-circuiting and air exchange between the pit and animal zones played important roles in the ventilation of the deep-pit swine finishing barn studied. Additional analysis is needed.