Technical Abstract: The objective of this experiment was to compare breeding date and overall pregnancy rates in beef cows given a single i.m. injection of 25 mg PGF-{2'}- (Lutalyse; control) on d 7 of a natural breeding season with or without 7-d pretreatment with an intravaginal progesterone insert (CIDR). Crossbred cows from two locations were stratified by age within location and randomly allotted to either treatment or control; CIDR were inserted on d 0 and removed on d 7; location 1:n=224, 3.36 ' .04 yr age, 62.5 ± 1.21 d postpartum (PP), 513.9 ± 4.6 kg BW, and 3.9 ± .04 BCS; location 2:n=74, 4.64 ' .15 yr age, 58.4 ± 1.17 d PP, 488.8 ± 6.4 kg BW, and 4.74 ± 0.57 BCS. Bulls were placed with cows in each herd from d 0 to d 60. Day of conception and cycle conceived (21 day periods) were estimated by ultrasonography on d 66 and 73; and again by rectal palpation on d 176 and 123 for locations 1 and 2, respectively. These response variables were analyzed with a model that included d PP, BCS, BW, and age at d 0 as covariates and location and treatment as fixed effects (location by treatment not significant). Days to conception (26.7 vs. 29.7 d for CIDR vs. control) and cycle (1.63 vs. 1.76 for CIDR vs. control) of conception tended (<i>P</i> < 0.13) to be decreased with CIDR. The number of pregnancies lost between first and second pregnancy diagnosis, did not differ (<i>P</i> = 0.31) between CIDR (7) and control (9). No beneficial affect of CIDR was observed (<i>P</i> = 0.23) for the cycle that cows became pregnant and remained pregnant. Overall pregnancy rates were not affected by treatment (<i>P</i> > 0.32) at either location 1 (99.6%) or 2 (97.3%). In conclusion, treatment with a CIDR and an injection of PGF-{2'}- was not effective in increasing overall pregnancy rates or advancing breeding date compared to cows receiving PGF-{2'}- on day 7 of the breeding season.