Ovulation-inducing factor (OIF) is a protein present in llama seminal plasma that has recently been identified as beta-Nerve Growth Factor (NGF) and it induces not only a high rate of ovulation but also appears to have luteotrophic properties in this species. A 2-by-2 experimental design was used to determine the effect of treatments (OIF/NGF vs GnRH) and categories of preovulatory follicle diameter (7-10 vs >10 mm) on ovulation rate, CL diameter and function in llamas. Llamas (n = 32 llamas per group) were randomly assigned to receive an intramuscular dose of: (a) 1 mg purified OIF/NGF in the presence of a follicle of 7-10 mm in diameter: (b) 50 mu g of GnRH in the presence of a follicle of 7-10mm in diameter: (c) 1 mg purified OIF/NGF in the presence of a follicle >10 ram in diameter; (d) 50 mu g of GnRH in the presence of a follicle >10 mm in diameter. Llamas were examined by ultrasonography every 12h from treatment to Day 2 (Day 0 = treatment) to detect ovulation, and again on Day 8 to determine CL diameter. Ovulation rates did not differ among groups. There was an effect of preovulatory follicle size on Corpus Luteum diameter at Day 8 (P < 0.001), however plasma progesterone concentration (n = 15/per group) was higher (P < 0.05) in the OIF/NGF - than that of the GnRH - treated group by the same day. We conclude that OIF/NGF treatment enhances CL function regardless preovulatory follicle size at the time of treatment. (C) 2014 Elsevier B.V. All rights reserved.