paired associative stimulation induces change in presynaptic inhibition of ia terminals in wrist flexors in humans enhancements in the strength of corticospinal projections to muscles are induced in conscious humans by paired associative stimulation pas to the motor cortex although most of the previous studies support the hypothesis that the increase of the amplitude of motor evoked potentials meps by pas involves longterm potentiation ltplike mechanism in cortical synapses changes in spinal excitability after pas have been reported suggestive of parallel modifications in both cortical and spinal excitability in a first series of experiments experiment 1 we confirmed that both flexor carpi radialis fcr meps and fcr h reflex recruitment curves are enhanced by pas to elucidate the mechanism responsible for this change in the h reflex amplitude we tested using the same subjects the hypothesis that enhanced h reflexes are caused by a downregulation of the efficacy of mechanisms controlling ia afferent discharge including presynaptic ia inhibition and postactivation depression to address this question amounts of both presynaptic ia inhibition of fcr ia terminals d1 and d2 inhibitions methods experiment 2 and postactivation depression experiment 3 were determined before and after pas results showed that pas induces a significant decrease of presynaptic ia inhibition of fcr terminals which was concomitant with the facilitation of the h reflex postactivation depression was unaffected by pas it is argued that enhancement of segmental excitation by pas relies on a selective effect of pas on the interneurons controlling presynaptic inhibition of ia terminals