Effects of 0.25 mg/kg + 0.25 mg/kg/h methadone (Me), 0.15 mg/kg + 0.1 mg/kg/h morphine (Mo), 25 μg/kg + 25 μg/kg/h butorphanol (B), and 1.5 mg/kg + 1 mg/kg/h tramadol (T) on heart rate (HR – 1A; bpm – beats/minute), systolic arterial pressure (SAP – 1B; mmHg), respiratory rate (RR – 1C; bpm – breaths/minute), core body temperature (T°C – 1D; °C), and gut sounds score (1E) of horses (n = 6) submitted to the experimental synovitis model. Values are reported as the mean ± SEM. Treatments were at 3 to 6 hours (in grey). The pilot study was conducted with 2 horses that received no opioid treatment (--- Trial). *Within each group, values differ significantly (P < 0.05) from the baseline value (0 minute). Values at the same time point in squares/rectangles indicate no significant difference (P < 0.05).