The reproductive response of 27 Boran and 20 Boran x Holstein cows to an exogenous superovulation treatment regimen was investigated. Upon cessation of treatment, cows were monitored to determine the onset and duration of estrus. Superovulatory response was by assessed via both transrectal palpation and ultrasonography. The overall proportion of cows that exhibited estrus was 77% and was higher (P=0.003) for Boran than for Boran x Holstein crosses. The mean interval from CIDR removal to the onset of estrus was 20.4 ± 1.8 hours, and the mean duration of estrus was 21.9 ± 1.1 hours The method (palpation vs ultrasound) used to determine the number of corpora lutea (CL) gave similar results (P>0.10; 8.5 and 8.6, respectively). Boran cows had a higher (P