\received DD MMMM YYYY \acceptedDD MMMM YYYY Background : The diagnosis and management of pituitary pars intermedia dysfunction (PPID) in horses is based on evaluating abnormal plasma concentrations of adrenocorticotrophic hormone (ACTH). Treatment commonly includes the oral dopamine agonist pergolide mesylate, which suppresses the pathologic overproduction of ACTH. Cabergoline is a commercially available long-acting injectable dopamine agonist that may provide an alternative means of treating horses with PPID by producing similar effects on plasma ACTH concentrations. Objectives: To evaluate the effects of IM cabergoline administration on plasma ACTH concentrations in horses with PPID Study Design: Experimental study with multiple repeated measures Methods: Twelve adult horses with PPID were randomly divided into treatment and control groups. The treatment group was administered a 0.01 mg/kg dose of intramuscular cabergoline at two time points, two weeks apart. Control horses received an intramuscular injection of sterile saline at the same time points. Horses were monitored for adverse effects. Blood samples were collected for endogenous ACTH concentration monitoring at serial time points. Results: Horses in the treatment group had a reduction in plasma ACTH concentration from a mean of 153.35 pg/mL to 24.117pg/mL following the initial administration of cabergoline. Their ACTH levels remained within the seasonally appropriate range until twelve days after the second cabergoline injection. Horses in the control group never achieved appropriate ACTH concentrations. The treated horses showed appetite suppression immediately following cabergoline administration, but no significant weight loss when compared to control horses. Main Limitations: The study group was small, and data collection overlapped with the natural fall rise in ACTH, which complicated interpretation of the results. Conclusions: Injectable cabergoline appeared effective in reducing plasma ACTH concentrations in the PPID horses to seasonally appropriate ranges. There appeared to be a transient reduction in appetite following cabergoline injection, but not a significant loss of weight in comparison to control horses.