During intracellular recording of the monosynaptic Ia EPSP (from muscle spindles) in lumbar motoneurones in adult mice we confirmed previous findings in the cat, that a test Ia EPSP is depressed following a train of conditioning stimuli to the same afferents. This phenomenon is referred to as Post Activation Depression (PActD). It is a long-lasting effect, and the size of the test Ia EPSP returns to normal values after 4–10 seconds. This has been explored indirectly in humans, measuring H-reflexes, and by direct measurements of the Ia EPSPs in cats. Reductions in PActD have been shown to be implicated in disorders involving spasticity. Given the development of transgenic mouse models of such disorders, we wanted to study the phenomenon in mice. Our primary aim was to see if it was possible to record PActD in adult mice in vivo using intracellular recording. Not only were we able to demonstrate that PActD is present in mice, but also that both the magnitude and the time course appear to be quantitatively similar to that which has been shown in cat. Using the SOD1G127X transgenic mouse model of the motoneurone disease Amyotrophic Lateral Sclerosis, of which spasticity is a symptom, we demonstrated significant reduction in PActD in this mutant, P = 0.0002 (using a 0.5 sec. interval between conditioning and test EPSP avoiding the effects of pre-synaptic inhibition).