Numerous mouth injuries have been reported in relation to bit use in horses. Rein tension is a potential risk factor, as it exerts pressure on the rostral parts of the mouth via the bit. The aim of this pilot study was to investigate rein tension during driving and its association with horse behaviour and mouth injuries. Eight horses were driven clockwise on a 1000 m oval racetrack for 3–4 km at walk and trot. Rein tension was measured with two IPOS rein sensors. Researchers filmed the horse’s behaviour from a vehicle moving alongside the horse on the outer track. The bit area of the mouth was examined before and after the session. One observer, blinded to the rein tension and mouth injury results, coded behaviours from videos according to a predefined ethogram. Median rein tensions in one rein ranged from 5 N to 36 N. Maximum rein tensions ranged from 106 N to 236 N. Five 30-second data subsets, representing samples of Low, Medium1, Medium2, High and Peak tension, were visually selected from the rein tension graphs to investigate behaviour differences during different rein tension levels. The durations of unrushed walk (P < 0.001) and mouth closed (P = 0.003) were longer in the Low rein tension subset. The durations of slow (P = 0.01) and fast trot (P = 0.003), mouth widely (P = 0.02) or repeatedly open (P = 0.03) were longer in the higher rein tension subsets. None of the horses had any prior mouth injuries in the bit area. After driving, three horses experienced moderate bruising. Their median rein tension was numerically higher (left 35 N; right 34 N) than that in horses without injuries (22 N; 18 N), and they showed longer periods of rushed walk (P = 0.04). Rein tensions were quite high, as previous studies suggest that horses find tensions above 6–10 N aversive. Horse trainers should monitor the horse's mouth behaviour, arousal state and ability to walk calmly and modify training accordingly, as mouth widely or repeatedly open at higher rein tensions may indicate aversion (mouth discomfort/pain) and rushed walk may indicate high arousal, which in turn may increase the risk for mouth injury.