Pain, atrophy and dysfunction of the longissimus dorsi in the equine back can lead to poor performance and altered biomechanics. Back problems are often treated by manipulative therapy to this muscle. The purpose of this study was to identify if manipulative therapy resulted in changes to muscle tone or electromyographic (EMG) activity immediately after treatment. We measured the muscle tone during standing using a mechanical tissue indenter and the EMG activity (both at the T16 level in the longissimus dorsi) during walking around a figure-of-eight course in 26 horses. The horses were randomly assigned into three groups that received: (a) spinal (McTimoney) manipulations, (b) reflex inhibition therapy or (c) a control group that was not manipulated. The muscle tone and activity were measured immediately after treatment. Both the McTimoney and the reflex inhibition groups showed significant decreases in muscle tone (c. 12%) and walking EMG activity (c. 19%). The control group showed no significant change in tone or EMG activity. These results document how the longissimus dorsi muscle responds immediately after manipulative therapy. Further studies would be needed to identify how long such changes persist or if such changes caused a reduction in pain or an increase in performance.