The heating system in the music wing of Fridley Junior High School in Fridley, Minnesota, was re-equipped with a CO2-controlled variable ventilation system. Under the new control strategy, the outdoor air dampers remained fully closed whenever the CO2-concentration within the conditioned spaces was below a specified setpoint. During the early months of 1980, the system operated in alternate periods under the conventional temperature-driven control strategy and the experimental CO2-driven control strategy. The performance of the system was monitored, and the subjective responses of occupants in the controlled spaces were obtained to determine whether they could detect differences in the environmental air quality resulting from the operation of the system under each control strategy. Results indicated that the use of this variable ventilation system may yield significant energy savings, while the occupants' subjective responses suggested the occupants felt warmer when the CO2-controlled system operated, despite no measurable difference in the space temperature under each control strategy. This feeling of warmth implies that a secondary energy savings may be accomplished by lowering the temperature setpoint of the space when the CO2-controlled system is in operation while maintaining comfort for the occupants.