Age alters the control of voluntary movement. A widely observed age-related adaptation is the heightened activation of the antagonist muscles during voluntary movements. We examined the possibility that age also modifies cortical reciprocal inhibition. In young (age 27, n=6) and old (age 73, n=6) adults a mild conditioning electrical stimulus was delivered to the median nerve at the elbow. The test stimulus, delivered by transcranial magnetic stimulation (TMS) at 1-ms increments between 11 and 24 ms after the electrical conditioning stimulus, evoked motor potentials (MEP) in the extensor carpi radialis (ECR) and flexor carpi radialis (FCR). The absolute TMS intensity, expressed as the percent of stimulator output, used to produce 1-mV control MEPs in the ECR was similar in young (mean 58.5, standard deviation ±12.8%) and old adults (60.3±20.3%, P=0.855). The size of the control MEP in the ECR was also similar in young (0.98±0.10 mV) and old subjects (0.90±0.14 mV, P=0.686). The age by conditioning interval interaction (P=0.001) showed that the MEPs in the ECR were significantly depressed at 14, 15, 16, 17, 18, and 19 ms (range 55.5--65.9% of control, all P<0.05) compared with control value of 100% and with old adults who showed no depression. The MEPs remained at control level in the FCR and were also unaffected in the first dorsal interosseus. These data confirm the existence of cortical reciprocal inhibition reported previously in young humans and show that age reduces this inhibition similarly to the previously reported reduction of spinal reciprocal inhibition reported in old adults. Activation of agonist and antagonist muscle pairs are most likely organized around a dual system of cortically and spinally mediated reciprocal inhibition that is altered by age. The data also indicate the need to use age-matched control subjects when comparing individuals with abnormalities resulting from disorders that occur at an old age.