The purpose of this study was to compare environmental conditions and mice in cages with four different mechanical ventilation designs and a static isolator cage. Environmental conditions (air velocity, temperature, relative humidity, bedding weight change, airborne dust, NH3, and CO2) were compared for each cage type (n = 5 per cage). Bedding type was chipped hardwood. Mouse response in each cage type was evaluated by body weight, feed consumption, water intake, location of specific behaviors, and building of bedding mounds. Commercial polycarbonate mouse caging units (29.2 x 19.1 x 12.7 cm shoebox style, stainless-steel round wire bar lids, and 7-cm-deep isolator cage filter tops) were modified to fit the mechanical ventilation cage types and were used for the static isolator cages. Mechanically ventilated cages were fitted with forced air inlets centered 5 cm above the cage floor on the 19.1 cm-side of the cage. Inlet air velocity was either 40 or 200 feet/min (n = 10 cages each), and the air volume exchange rate was 9.3 L/min. In half of the mechanically ventilated cages, the exhaust air was forced through a filter in the isolator cage top, whereas in the remaining mechanically ventilated cages, the air was forced through a single exhaust port fixed in the narrow side of the cage top directly above the air inlet. Inlet air velocity but not exhaust design affected intracage air velocity distribution. Other environmental conditions were similar between the four mechanical ventilation designs. Relative to the mechanically ventilated cages, the static isolator cages had lower air velocities, higher relative humidities, higher NH3 levels, higher CO2 levels, lower body weight gain, and lower water consumption; temperatures, particulate levels, and feed consumption rates did not differ significantly between cage types. Locations of bedding mounds and behaviors were similar in all cage treatments.