Objective: To evaluate the effectiveness of norgestomet Crestar? for synchronization of oestrus and its effect on pregnancy rate in cows. Methods:after a waiting period (~ 90 days postpartum), Holstein-Frisian females kept in different dairy farms were inseminated for the detection of estrous behavour (n=133). Females were allocated randomly to treated (n=69) or control (n=64) groups. The treated animals received the Crestar? treatment during 9-10 days. After implant removal, the treated animals were artificially inseminated (AI) for the detection of standing estrous behaviour. Blood samples were collected on Days 0, 21 and 30 after service. Plasma progesterone (P4) and pregnancy-associated glycoprotein (PAG) levels were determinate by radioimmunoassay. Results:the study showed a higher rate of heat synchronization in the treated group (94%) compared to the control group (86%). Out of 133 cows inseminated, thirteen (4 treated and 9 control) had a plasma progesterone concentrations higher than 1.0 ng/mL at service. The plasma PAG levels at Day 30 after AI were significantly lower in the treated group (7.13 ± 2.29) compared to the control group (8.37 ± 2.30). Pregnancy rate was 28% (16/57) and 42% (23/54) for treated and untreated group, respectively. There were no significant differences in the pregnancy rates of different parity in both groups. Conclusion:The norgestomet Crestar? treatment carried out during 9-10 days did not improve reproductive performance in dairy cattle. However, these data demonstrate that this hormonal protocol can be used to obtain high oestrus cow rates to initiate correct timing of insemination.