From 1973 to 1977 a series of laboratory tests involving almost 3000 people were conducted to determine the factors that contribute to the thermal comfort of automobile passengers while using air conditioning under summer heat loads. Four studies will be reviewed. In the first study, 2200 subjects were exposed for 45 min. to an environment of 110°F/40% in a 1973 Ford Vehicle buck for the purpose of evaluating the effects of the register size, the air flow rate and the discharge air temperature on comfort. The results showed that while the register size does not affect the time to reach a comfortable condition, the time to reach comfort in the front seat varies from 4 minutes with an air flow of 400 cfm (50°F discharge air at 10 minutes) to 18 minutes with 150 cfm (60°F discharge air); in the rear seat, the corresponding times were 8.5 and 39 minutes. In the second study designed to define the criteria for comfort during a prolonged stabilized period after a transient cool-down, a simulated 20 mph 30 minute cool-down followed by a 60 minute 40 mph steady-state condition was simulated when the chamber environment was 110°F/40% RH; during the third study that followed, the environment was 90°F/70% RH. The results demonstrated that the average time to reach comfort was reduced by 55% when the temperature was reduced from 110°F to 90°F; at 90°F the blower speed was changed more frequently by a factor of 3; the register vane setting was changed the same number of times in both studies; and the average time the front seat passengers deflected the register plumes off themselves was the same in both conditions. Because a difference was uncovered in the results of the cool-down tests, a fourth study was conducted to determine if the difference in the comfort response was dependent on seasonal acclimation. In tests conducted in the summer and the winter, the subjects tested in the winter exhibited a thermal sensation response that was higher than the subjects who were tested in the summer and the time required for the subjects who were tested in the winter to reach a condition of comfort was longer than for the subjects who were tested in the summer.