Remifentanil is a potent opioid with minimum alveolar concentration–sparing effects in humans and dogs, but its effects in horses have not been reported. We proposed to evaluate the cardiopulmonary effects, the duration, and recovery quality of remifentanil in isoflurane-anesthetized horses. Ten horses were premedicated with xylazine (1 mg/kg IV), induced with ketamine and diazepam (2.2 mg/kg/0.1 mg/kg IV), and maintained with isoflurane in oxygen delivered by a mechanical ventilator. Each horse was anaesthetized twice in a randomized order, with 1 week between treatments. In both treatment groups, a bolus of 0.5 μg/kg and a 120-minute infusion of remifentanil (0.375 μg/kg/min) or sterile water (delivered at the same rate per volume as the remifentanil) commenced when an end-tidal isoflurane concentration of 1.2%–1.4% was reached. Cardiopulmonary variables, arterial blood gas variables, time to extubation, and recovery scores were not significantly different between groups. The time to first movement was significantly different, which is longer for the placebo group (25.6 ± 17.7 minutes) than that for the remifentanil group (11.33 ± 5.38 minutes). The time to standing was significantly different; in the placebo group, it was 47 ± 20 minutes, and in the remifentanil group, it was 30.7 ± 9.3 minutes.