Equine respiratory patterns during swimming were examined in five normal horses. The experiment included a preliminary warming-up stage and 6 circuits of swimming around an annular pool of a 50-meter-circumference. The horses were examined for respiratory rates, intratracheal pressures, inspiratory time (TI), expiratory time (TE), respiratory cycle (T; TI + TE), heart rates, blood lactate concentrations, hematocrit and blood gases. The respiratory rates were maintained around 25/min. Blood gas values changed significantly during swimming. The intratracheal pressures during expiration and inspiration increased significantly with exercise duration compared to the immediately after the warming-up stage. The duty ratio (TI/T) averaged 0.33, which implied that the expiratory time was roughly doubled the inspiratory time. We considered that a longer expiratory time may limit sudden collapse of airways by water pressure during swimming and prevent a radical decrease of air space volume, thus maintains buoyancy.