This study was carried out to compare effects on the pregnancy rates achieved by timed AI (TAI) of cosynch, ovsynch and cosynch+ovsynch protocols in lactating dairy cows (60 d postpartum) and heifers (16-18 m old) in which the state of estrus is not determined. In group I (cosynch) cows (n: 97) and heifers (n: 43) were treated with an intramuscular. injection of synthetic GnRH (10 mu g) on day 0.7 d after the GnRH injection they were given an i.m. injection of PGF(2)alpha (25 mg) and a second injection of GnR-H (10mg) 48 h after the PGF(2)alpha injection. All cows and heifers were served by TAI at the second GnRH injection. In group 11 (ovsynch) cows (n: 94) and heifers (n: 43) were treated as in group 1, but TAI was performed about 16 h after the second GnRH injection. In group III (cosynch+ovsynch) cows (n: 95) and heifers (n: 43) were treated as in group 11 and 1, but double TAI was performed. The first TAI was administered at the time of the second GnRH injection and the second TAI was administered about 18-20 h later. Pregnancy was diagnosed by rectal palpation after similar to 45 d following AI. Pregnancy rates for heifers in group III were significantly greater than in group 1. The pregnancy rate was numerically greater for cows in group 111, but it was not significantly different from those in group I and II. The effect of treatment on the total pregnancy rate was similar in all three treatment groups. However, the total pregnancy rate was higher in group III than in group I and II. Combining pregnancy results of all treatment groups, the pregnancy rate for heifers with a corpus luteum (CL) was significantly greater than without a CL. In summary, in dairy herds which have poor detection of estrus, in unobserved estrous dairy heifers treated with a GnRH-PGF(2)alpha-GnRH protocol and double TAI can achieve significantly greater pregnancy rates than those using the cosynch procedure. In addition, pregnancy rates in heifers can probably be improved by more careful time of the second AI.