The efficacy of the use of controlled internal drug releasing (CIDRR) device containing 1.9g progesterone in inducing estrus synchronization of heifers in Japan was first attested. Eighty-eight Holstein heifers were randomly designated for the 7, 12, and 14 days of CIDRR insertion periods. The effects of CIDRR with and without estradiol-benzoate (E2) and prostaglandin F2α (PGF2α) at 7, 12 and 14 days of insertion periods were evaluated and compared in terms of progesterone concentrations during the 1st and 2nd day of insertion periods, at CIDRR removal and at the AI time; retention rates, estrus incidence, and pregnancy rates.An overall CIDRR device retention rate of 85.2% (75/88), estrus incidence of 90.7% (68/75) and pregnancy rate of 63.3% (38/60) were recorded. Specifically, estrus incidence and pregnancy rates in both 7 days (86.8 and 60.7%) and 12 days (93.3 and 61.5%) insertion groups showed encouraging results. No significant difference was found between treatment groups. However, the 14 days insertion period without the use of additional hormones showed the highest pregnancy rate of 83.3% (5/6). An E2 capsule implanted at insertion and PGF2α injected at CIDRR removal showed no effect on pregnancy rate, but the incidence of 'standing' estrus increased significantly significantly (p<0.05) with PGF2α treatment.