One of the causes of poor fertility in high producing dairy cows is inadequate progesterone. Therefore, we determined the efficacy of an intravaginal insert containing 1.55 g of progesterone (PRID) given before and/or after timed AI (TAI) on ovarian response, plasma progesterone concentrations, pregnancy per AI (P/AI) and pregnancy losses. Lactating dairy cows at three locations were assigned (Day 0) to an Ovsynch protocol with (N = 294) or without (N = 314) a PRID. The Ovsynch protocol consisted of two injections of 100 μg gonadorelin (GnRH) 9 days apart and one injection of 500 μg cloprostenol (PG) 7 days after the first GnRH treatment. Insertion and removal of PRID occurred concurrent with the first GnRH and PG treatments, respectively. Timed AI was carried out 12 to 16 hours after the second GnRH. Ovarian status of a subset of 217 first service cows had been presynchronized with 2 treatments of PG 14 days apart with the last PG given 12 days before the first GnRH of the Ovsynch protocol. Body condition score (scale of one to five) was recorded at TAI. Ultrasonographic examinations were done in all cows at first GnRH, at PG, at TAI, and 24 hours after TAI for response to treatment and at 32 and 60 days after TAI for confirmation of pregnancy. At 4.5 days after TAI (Day 14), cows that responded to PG and ovulated after the second GnRH treatment were reassigned to receive (N = 223) or not receive (N = 229) a PRID for 7 days. Blood samples were taken for progesterone determination at PG treatment, at TAI, and post TAI on Days 14 and 21. The PRID treatment pre-TAI reduced the percentage of cows ovulating before TAI (5.8% vs. 11.1%), and significantly increased P/AI in nonpresynchronized cows (41.3% vs. 25.1%). Cows ovulating in response to the first GnRH treatment, cyclic cows, and cows with body condition score of 2.75 or more had increased P/AI, but the addition of a PRID pre-TAI to these cows did not increase P/AI. The PRID treatment post TAI did not affect P/AI, but reduced pregnancy losses (6.1% vs. 11.4%) between 32 and 60 days of gestation. The reduction in pregnancy losses tended (P = 0.10) to be significant in acyclic cows receiving a PRID than in those not receiving a PRID (5.6% vs. 33.3%). Plasma progesterone concentrations at PG treatment and on Day 21 (11.5 days after TAI) were linearly associated with P/AI. In conclusion, progesterone supplementation pre-TAI increased P/AI in nonpresynchronized cows. Progesterone supplementation post TAI reduced pregnancy losses, particularly in acyclic cows.