Clonidine and rilmenidine are drugs used in human medicine as central acting antihypertensive agents due to their actions on the alpha2-adrenoceptor and I1 imidazoline receptors in the central nervous system. Currently, clonidine is also used as a pre-anesthetic medication and in spinal anesthesia, and rilmenidine, despite the lower selectivity for alpha2-adrenergic receptors than clonidine, has also shown antinociceptive potential in experimental pain models. In this study, six horses were submitted to four treatments: R1 group (0.014 mg/kg of rilmenidine); R2 group (0.021 mg/kg of rilmenidine); Clo group (0.002 mg/kg of clonidine), and a control group. The assessment comprehended their heart and respiratory rates, systolic blood pressure and intestinal motility at basal levels and, then, 60 and 120 minutes after the oral administration of the drugs. Rilmenidine decreased heart rate on the two tested doses, which did not occur in the Clonidine treatment; slight variations in systolic blood pressure in all treatments and respiratory rate reduction in treatments with rilmenidine at 0.021 mg/ kg and clonidine at 0.002 mg/kg were also observed. Further studies with different dosages and varied administration routes are still necessary in order to elucidate more effects of these drugs on horses. © 2016, Cienc. anim. bras., Goiânia. All rights reserved.174608614