Two trials were conducted to characterize the performance of an automated electronic activity tag system as an aid for detection of estrus in dairy cattle. In trial 1, activity tags were attached to the rear leg of 24 cows at approximately 35 d postpartum and remained attached until pregnancy. Data collection included 66 periods of predicted estrus (cyclic periods of 18 to 24 d prior to the date of pregnancy). A summary of the data recorded in the activity tags was transmitted telemetrically to a personal computer at each milking via stationary antenna in the milking parlor. An electronic flag used an increased activity ratio to determine the ratio of activity in a test period of the previous 12 h to activity in the same 12 h during the 2 d previous to the test period, thus indicating estrus. Activity patterns were characterized from data recorded on the tags at 2-h intervals. The tag detected 74% of predicted periods of estrus versus 58% reported by herders. An increased activity ratio for at least 4 consecutive h reduced false-positive designations. In trial 2, activity tags were attached to front and rear legs on five cows, and activity patterns from the two sites were compared for 2 mo. Patterns of activity were similar from tags attached at either site, and the sites were not different in their discrimination between periods in which estrus did or did not occur. The activity tag system was an effective practical tool to detect estrus.