A two-part investigation was conducted as part of a continuing program at Langley Research Center to account for the variability in response of people to aircraft noise. Earlier studies within this program indicated a systematic and substantial increase in the annoyance of people to aircraft noise during a day of testing. Logically, this increase could be attributed to either a time-of-day effect, i.e., a circadian rhythm effect, or simply to increases in the number of aircraft noise exposures during the test period. In order to investigate the time factor, thresholds of annoyance to noise were obtained from 72 subjects at home as they went about their daily activities. These thresholds were obtained with cassette tape recorders/headsets every 2 h over a 24-h period. The effect of number of aircraft noise exposures was investigated using the same subjects, but within the laboratory situation. In these tests, thresholds of annoyance to noise were obtained from subjects after exposure to varying numbers of aircraft exposures. The threshold of annoyance data over 24 h indicated subjects showed only a small increase of sensitivity to noise between 10 p.m. and 6 a.m. Consequently, these data cannot explain response variation to tests conducted during the daytime. However, the number of aircraft exposures in a laboratory influences subject sensitivity and accounts for this type of test period variation.