The objective of the experiment was to examine the necessity of adding a GnRH injection to a 14-d controlled internal drug release (CIDR)-based protocol for synchronization of estrus in beef heifers that were prepubertal or estrous-cycling at the initiation of treatment. The hypothesis tested was that the addition of GnRH in a CIDR-based estrus synchronization protocol would increase the synchrony of estrus after PGF(2alpha) (PG). Beef heifers (n = 285) were assigned to 1 of 2 treatments within reproductive tract scores (2 or 3 = prepubertal; 4 or 5 = estrous-cycling) by age and BW. Heifers assigned to CIDR Select received a CIDR insert (1.38 g of progesterone) from d 0 to 14 followed by GnRH (100 microg, intramuscularly) on d 23 and PG (25 mg intramuscularly) on d 30. Heifers assigned to CIDR-PG received a CIDR insert from d 0 to 14 and PG on d 30. Heifers were fitted with a HeatWatch estrus detection system transmitter at the time of PG administration for continuous estrus detection during the synchronized period (0 to 144 h after PG); AI was performed 12 h after estrus onset. Estrous response did not differ (P = 0.43) between treatments (94% CIDR Select, 98% CIDR-PG). Mean interval to estrus after PG was 7 h shorter (P = 0.01) and variance for interval to estrus was reduced (P < 0.01) among CIDR-PG-treated compared with CIDR Select-treated heifers. Conception rate to AI tended (P = 0.09) to be greater for CIDR-PG heifers (67%) compared with CIDR Select heifers (58%), and AI pregnancy rate was greater (P = 0.05) for CIDR-PG heifers (66%) compared with CIDR Select heifers (55%). Final pregnancy rate at the end of the breeding season was similar for the 2 treatments (81% for both; P = 0.94). We conclude that the administration of GnRH 9 d after CIDR removal in the CIDR Select protocol is not required to facilitate an improvement in the synchrony of estrus in beef heifers.