Llamas require a copulation stimulus to elicit an LH surge that subsequently leads to induced ovulation. Gonadotropin-releasing hormone has been used to induce ovulation in llamas; however, results have been variable because of the different analogues and doses in previous studies (Bravo PW et al. 1992 Biol. Reprod. 47, 884-888; Ratto M et al. 2006 Anim. Reprod. Sci. 91, 299-306). Additionally, the effect of different doses and types of GnRH analogues on CL formation and function has not been described in detail. The purpose of the present study was to conduct a dose response trial to examine the effect of a synthetic analogue of GnRH, gonadorelin acetate (Ovalyse^®, Pfizer, Santiago, Chile) on ovulation and CL formation in llamas. Nonpregnant and nonlactating adult llamas (range 4-8 years) weighing 90 to 130 kg were examined once daily by transrectal ultrasonography using a B mode scanner with a 7.5-MHz linear-array transducer (Aloka^® SSD 500, International Clinics, Santiago, Chile). Llamas with a follicle ≥8 mm in diameter that had grown for 3 consecutive days were assigned randomly to be given an i.m. administration of gonadorelin at a dose of 1) 50 μg (n = 5); 2) 25 μg (n = 8); 3) 12.5 μg (n = 9); 4) 6.25 μg (n = 8), or 5) i.m. administration of 0.5 mL of PBS as a negative control group (n = 5). Llamas were examined by ultrasonography every 12 h from treatment (Day 0) to ovulation and thereafter on Day 8 to determine CL formation. Ovulation was defined as the sudden disappearance of a large follicle ≥8 mm that was detected during the previous examination and was confirmed by subsequent CL formation on Day 8. Ovulation rate was analyzed by chi-square analysis. Follicle diameter (at the time of treatment), interval from treatment to ovulation, and CL diameter on Day 8 were analyzed using one-way ANOVA. Mean ± SD diameter of preovulatory follicle did not differ (P = 0.4) among groups (11 ± 1.6, 10.6 ± 1.5, 11.5 ± 1.9, 11.7 ± 1.9, and 9.8 ± 2.9 mm for 50, 25, 12.5, and 6.25 μg of gonadorelin and 0.5 mL PBS, respectively). The proportions of ovulated llamas did not differ (P = 0.1) among GnRH-treated groups (100, 62.5, 77.7, and 62.5% for 50, 25, 12.5, and 6.25 μg of gonadorelin, respectively) but they did differ (P < 0.01) from the control group (0%). Also, the interval from treatment to ovulation in those ovulated females ranged from 36 to 48 h and it did not differ (P = 0.6) among groups. Corpus luteum diameter on Day 8 (12.2 ± 1.8, 10.2 ± 1.1, 11.7 ± 2.2, and 11.3 ± 1.8 for 50, 25, 12.5, and 6.25 μg of gonadorelin, respectively) did not differ (P = 0.3) among groups. In conclusion, the minimal dose of gonadorelin used in this study (1/8 dose) was as equally effective as the highest dose to induce ovulation in llamas without compromising CL formation.