We investigated whether the analgesic mu-opioid fentanyl can be used safely in dogs in everyday clinical veterinary practice, with limited and non-invasive monitoring. To this end, the cardiorespiratory effects of fentanyl, administered in doses reported to be adequate for inducing opiate analgesia in spontaneously breathing canine patients, were evaluated by measuring the respiration rate, oxygen saturation (SpO2), heart rate, respiratory sinus arrhythmia (RSA), and rectal body temperature. Ten Beagle dogs, all spontaneously breathing room air, underwent three separate sessions in which they received in random order either saline, fentanyl 5 microg/kg/h or fentanyl 10 microg/kg/h. Each session started with a non-medication period, followed by acepromazine with glycopyrrolate, followed by a loading dose and infusion of saline or fentanyl, and ended with the administration of the antagonist naloxone. At the doses studied, fentanyl did not significantly change the respiration rate or have a clinically relevant effect on SpO2 or RSA, whereas it significantly decreased the heart rate and core body temperature. In the dose range tested and under the conditions described in this protocol, we conclude that fentanyl can be safely administered to healthy dogs spontaneously breathing room air.