We analysed somatosensory evoked potentials (SEPs) in the patients with juvenile myoclonic epilepsy (JME) in order to find out if sodium valproate (VPA) affects SEP latencies and amplitudes. SEPs were studied in 23 patients with JME receiving VPA monotherapy, eight patients with JME not receiving VPA, and a control group consisting of 20 healthy subjects. The N20, P24, and N34 latencies bilaterally were significantly prolonged in the JME group receiving VPA as compared with the control group. In the untreated patients the P24, and N34 latencies bilaterally and the N20-P24 interpeak latency on the right, were significantly prolonged as compared with the control group. In addition, in the patient group without treatment, the N20-P24 amplitudes bilaterally and the P24-N34 amplitudes from left sided median nerve stimulation, were greater as compared with the control group. In the SEP latencies, no significant differences were observed between the patients treated and untreated with VPA. Regarding SEP amplitudes, in the untreated group, while the N20-P24 amplitudes from right sided median nerve stimulation were significantly enhanced, all the other amplitudes also showed increase, even insignificant, as compared with the patients treated by VPA. These findings suggest that the SEPs latencies are prolonged, and the amplitudes are enhanced in JME. The changes of the SEPs latency in JME could be due to abnormal synaptic transmission and not influenced by VPA. On the other hand, the increase of the amplitude tends to be lowered by VPA.