To determine the accuracy of an activity monitoring system (AMS) for detecting a synchronized estrus, nulliparous Holstein heifers (n = 33) 13 to 15 mo of age and housed in the same pen were fitted with AMS neck collars (Heatime; SCR Engineers Ltd., Netanya, Israel). A total of 2 AMS data transceiver units were located over each water trough in the pen. All heifers received 3 sequential PGF_2α treatments (25 mg dinoprost tromethamine; Zoetis, Parsippany, NJ) at 14 d intervals. Activation of pressure-activated Heatmount devices (Kamar Heatmount Detectors; Kamar Inc., Steamboat Springs, CO) affixed to individual heifers was used as a gold standard to determine the day of standing estrus after PGF_2α treatment. Ovaries of all heifers were evaluated using transrectal ultrasonography on the day of PGF_2α treatment, on the day of standing estrus, and 7 d later to confirm ovulation. Blood samples collected from all heifers on the day of PGF_2α treatment and on the day of standing estrus were assayed for progesterone (P4) by RIA. Total true standing estrus periods (n = 94) after the 3 PGF_2α treatments was defined when a heifer had a follicle >10 mm in diameter and P4 > 1 ng/mL on the day of PGF_2α treatment, P4 < 1 ng/mL on the day of estrus, and P4 > 1 ng/mL and ovulation of a follicle 7 d after PGF_2α treatment. Data were analyzed using the GLIMMIX, MIXED, and FREQ procedures of SAS. Overall, 72% (68/94) of true estrus periods were detected by the AMS. When true estrus periods were distributed based on the day of standing estrus after PGF_2α treatment, sensitivity of the AMS was 50%, 93%, 77%, and 47% at 1, 2, 3, and ≥4 d after PGF_2α treatment. Diameter of the largest follicle (mm) on the day of estrus tended to increase (P = 0.10) as day of estrus after PGF_2α treatment increased (d 1 = 13.4 ± 0.3; d 2 = 13.6 ± 0.3; d 3 = 13.7 ± 0.3; ≥ d 4 = 14.4 ± 0.6 mm). We conclude that the AMS system was partially effective at detecting estrous activity after synchronization of estrus using PGF_2α in nulliparous Holstein heifers. Supported by USDA NIFA Hatch project 1006519