The relationship between aging, physical activity, and vagal influence on the heart was assessed by measuring resting heart period variability in postmenopausal women. Participants were 14 aerobically trained women (mean age 55 ± 1.0 years) and 20 untrained women (mean age 59 ± 1.1 years). Participants lay for 25 min while heart period variability was assessed during spontaneous and paced breathing (7.5 breaths·min-1). Heart period variability was assessed through time series analysis (HPVts) of the interbeat interval. Results indicated that the trained women had significantly (p < .05) lower supine resting heart rate than the untrained group. HPVts at high frequencies during spontaneous and paced breathing was greater for trained compared to untrained participants. Similarly, HPVts at medium frequencies during spontaneous and paced breathing was greater for trained compared to untrained participants. Also, rate pressure product of the trained group was significantly lower than for the untrained. These results extend prior research by showing that aerobically trained postmenopausal women possessed significantly elevated resting vagal influence on the heart compared to their untrained counterparts. © 1998 Human Kinetics Publishers, Inc.