This study investigates the relationship between ambient fine particle pollution and impaired cardiac autonomic control in the elderly. Heart rate variability ( HRV ) among 56 elderly ( mean age 82 ) nonsmoking residents of a retirement center in Baltimore County, Maryland, was monitored for 4 weeks, from July 27 through August 22, 1998. The weather was seasonally mild ( 63 ± 848F mean daily temperature ) with low to moderate levels of fine particles ( PM 2.5 < 50 g / m 3 ) . Two groups of approximately 30 subjects were examined on alternate days. A spline mixed -effects model revealed a negative relationship between outdoor 24 -h average fine particulate matter ( PM 2.5 ) and high -frequency ( HF ) HRV that was consistent with our earlier Baltimore study for all but 2 days. These 2 days were the only days with significant precipitation in combination with elevated PM 2.5 . They were also unusual in that back -trajectory of their air masses was distinctly different from those on the other study days, emanating from the direction of rural Pennsylvania. Mixed -effects analysis for all 24 study days showed a small negative association of outdoor PM 2.5 with HF HRV ( À 0.03 change in log [ HF HRV ] for a 10 g / m 3 increment in PM 2.5 ) after adjustment for age, sex, cardiovascular status, trend, maximum temperature, average dew point temperature, random subject intercepts, and autocorrelated residuals. After excluding study days 4 and 5, this association was strengthened ( À 0.07 change in log [ HF HRV ] for 10 g / m 3 PM 2.5 , 95% CI À 0.13 to À 0.02 ) and was similar to that obtained in an earlier study ( À 0.12 change in log [ HF HRV ] for a 10 g / m 3 increment in outdoor PM 2.5 , 95% CI À 0.24 to À 0.00 ) [ Liao D., Cai J., Rosamond W.D., Barnes R.W., Hutchinson R.G., Whitsel E.A., Rautaharju P., and Heiss G. Cardiac autonomic function and incident coronary heart disease: a population -based case -cohort study. The ARIC Study. Atherosclerosis Risk in Communities Study. Am J Epidemiol 1997: 145 ( 8 ) : 696 ± 706 ] . Acute ( 1 to 4 h ) previous PM 2.5 exposure did not have a stronger impact than the 24 -h measure. A distributed lag model incorporating the six preceding 4 -h means also did not indicate any effect greater than that observed in the 24 -h measure. This study is consistent with earlier findings that exposures to PM 2.5 are associated with decreased HRV in the elderly.