The aim of this study was to evaluate the effects of pre-anesthetic use of clonidine on time-domain heart rate variability (HRV) and arterial blood pressure in healthy anesthetized dogs. Six healthy adult mixed-breed dogs were administered a clonidine (clonidine group, CLG) and 30 days later, a placebo (control group, CG) preanesthetic protocol, in addition to propofol, isoflurane, and an bolus of tramadol and the continuous infusion thereafter. The total time mean values of HRV meanNN, SDNN, SDANN, SDNNI, and rMSSD were higher in the CL group, as observed in some HRV variables on tramadol bolus time (T4), tramadol continuous infusion (T8), and tracheal extubation time (T10). No significant differences in arterial blood pressure were observed, however, two dogs had a second-degree atrioventricular block (Mobitz II) at the tramadol bolus time (T4). These results led us to conclude that the clonidine anesthetic protocol resulted in sympathetic outflow block and an increase in parasympathetic tone, without significant effects on blood pressure. Notably, cardiac electrical disturbance in two dogs in the CL group. Although the pre-anesthetic use of clonidine in dogs with fear-based behavioral problems should be considered, its association with tramadol should be avoided or carried out with caution owing to the existing cardiovascular risk.