Aali, M., Pretheeban, T., Giritharan, G. and Rajamahendran, R. 2008. Pregnancy rates and peripheral progesterone levels following Ovsynch or CIDR ovulation synchronization/timed artificial insemination protocols in postpartum dairy cows. Can. J. Anim. Sci. 88: 457Á461. Peripheral progesterone (P 4 ) concentrations as well as pregnancy rates (PR) between cows treated with Ovsynch or CIDR ovulation synchronization/TAI protocols were compared. Two hundred and twenty-seven postpartum lactating Holstein cows were randomly assigned to Ovsynch (n 0111) or CIDR (n 0116) ovulation synchronization/TAI protocols. Pregnancy was diagnosed by ultrasonography at day 35 after TAI and later confirmed by rectal palpation at day 60 post TAI. Milk samples were taken from the beginning of treatment (day (10) until day 35 after TAI (day 0) for P 4 determination. Pregnancy rates for Ovsynch and CIDR groups of cows were 31 and 41%, respectively. The PR of ovulation synchronized cows for Ovsynch and CIDR groups were 49 and 69%, respectively. Progesterone levels during the sampling period were similar (P0.05) between Ovsynch and CIDR treated cows.