The objective of this study was to determine the effect of leptin administration on the development, vascularization and function of Corpus luteum (CL) in alpacas submitted to pre-ovulatory fasting. Fourteen alpacas were kept in fasting conditions for 72 h and received five doses of o-leptin (2 mu g/kg e.v.; Leptin group) or saline (Control group) every 12 h. Ovulation was induced with a GnRH dose (Day 0). The ovaries were examined every other day by trans-rectal ultrasonography (7.5 MHz; mode B and power Doppler) from Day 0 to 13 to determine the pre-ovulatory follicle diameter and ovulation, and then to monitor CL diameter and vascularization until the regression phase. Serial blood samples were taken after GnRH treatment to determine plasma LH concentration; and every other day from Days 1 to 13 to determine plasma progesterone and leptin concentrations. The pre-ovulatory follicle and CL diameter, LH, progesterone and leptin plasma concentrations were not affected by treatment (P > 0.05). The vascularization area of the CL was, nevertheless, affected by the treatment (P < 0.01) with significant differences between groups at Days 3, 7 and 9 (P < 0.05). The Leptin group had a larger maximum vascularization area (0.67 +/- 0.1 compared with 0.35 +/- 0.1 cm(2); P < 0.05). In addition, there was a positive correlation between CL vascularization, CL diameter and plasma progesterone. The exogenous administration of leptin during pre-ovulatory fasting increased the vascularization of the CL in alpacas in vivo.