The ovarian renin-angiotensin system may play an important role in follicular growth and maturation, as well as in the process of ovulation. The aim of this study was to investigate the effects of administration of a GnRH analogue to cows with ovarian follicular cysts on plasma renin concentrations and ovarian vascularization. This study was performed with 60 Friesian cows, which were diagnosed with follicular cysts, and randomly allocated into two groups: group A (treatment; n=30) received 2ml of lecirelin (Dalmarelin((R)) - Fatro), per head via sacro-coccygeal epidural, and group B (control; n=30) received 2ml saline solution (0.9% NaCl) per head by the same route. Blood samples were immediately collected prior to administration (T0) and then 24h (T1), 48h (T2) and 8 days (T3) after administration of the treatment, for both groups. Ovarian vascularization was evaluated utilizing Power Doppler on these same days in 10 animals from each group. The number of pixels detected by Power Doppler was used as an indicator of the degree of vascularization. Plasma renin concentrations remained relatively constant for the control (group B) animals, but increased as the sampling period progressed (NS) for the treated cows (group A). Similarly, there were no changes in ovarian vascularization (number of pixels) for the control cows, but vascularization increased throughout the sampling period in the treated animals. The number of pixels associated with cysts was significantly higher for treated compared to control cows at 24h after treatment (P<0.001). The epidural administration of a GnRH analogue was determined to be a highly effective therapy for follicular cysts (regression occurred in 82% of treated cows within 8+/-2 days after treatment, but in none of the control cows), which also enhanced ovarian vascularization.