Oxytocin is used increasingly in mares to prolong corpus luteum (CL) function to suppress estrus. When mares develop prolonged CL function in response to oxytocin treatment, the CL generally remains functional for 60–90 days. However, a longer duration of CL function can make this method of estrus suppression even more clinically useful. We hypothesized that human chorionic gonadotropin (hCG) treatment during the period of oxytocin-induced prolonged CL function would extend the duration of CL function (i.e. beyond 90 days). Prolonged CL function was induced in mares (n = 14) by treating them with 60 units of intramuscular oxytocin once daily on days 7–14 after ovulation. Mares were then randomly assigned equally to a control group that received no additional treatment and an hCG-treated group that received 2,500 units of intramuscular hCG on days 30, 45, 60, 75, and 90 after ovulation. Jugular blood samples were collected for progesterone concentration determination on the day of ovulation and then 3 times (M, W, and F) weekly for 120 days. Duration of CL function was not different (p > 0.05) and it was 78.0 ± 7.5 and 91.4 ± 20.4 days (mean ± standard deviation [SD]) in control and hCG-treated mares, respectively. Therefore, hCG treatment (during the period of oxytocin-induced prolonged CL function) did not extend CL function, hence alternative methods should be explored.