Threshold electrical stimulation of several widely dispersed thalamic and extrathalamic nuclei ( i.e., rostral CM, VM, lateral RE, VL, ventral MD, and the main part of the Cd nucleus) elicited spindles only on the anterior sigmoid gyrus, while threshold stimulation of other nuclei ( i.e., CL, Pc, VA, medial RE, LP, dorsal MD, and the posterior Cd) elicited spindles only on the suprasylvian gyrus. These findings replicated the observation that spontaneous spindles often appeared independently on these two gyri and suggested that each group of nuclei may be the source of ‘synchronizing influences’ on activity at the anterior sigmoid and suprasylvian gyri. Further findings were interpreted as indicating that the nuclei in each group may be linked together in two neural systems identified as the ‘ventral’ spindle system, associated with the sigmoid gyrus, and the ‘dorsal’ spindle system,associated with the suprasylvian. The supportive findings were that stimulation of a nucleus elicited spindles which were relatively localized to other nuclei in the same group. In addition, these elicited spindle waves were in relative coincidence, or synchrony, when recorded simultaneously in two nuclei from the same group but had a low coincidence when recorded in one nucleus from each group. Finally, the spinles reticular arousal stimulation than the same spindles recorded in the other group of nuclei.