Multisecond oscillations (mean: 20 - 30 sec) in neuronal firing rates in GP, substantia nigra pars reticulata, subthalamic and thalamic reticular nuclei in immobilized rats are robustly modulated by DA. These oscillations in firing rate are also correlated with bursts of transcortical EEG (tEEG) theta (4 - 7 Hz) rhythm (SFN 26: 960, 962). GP firing rate periodicities do not correlate with the tEEG theta rhythm itself, but with the multisecond periodicities in theta power. To explore the source of the correlated tEEG theta signal, localized field potentials in the HP and MCX were recorded simultaneously with GP unit activity. Dorsal HP field potentials exhibited bursts of theta power that correlated with multisecond oscillations in GP firing rates (12/23). MCX field potentials infrequently showed this correlation (4/23). Following DA agonist administration, the incidence of correlated oscillations in GP firing rate and HP theta bursts remained high (13/18) and the power of these oscillations was significant...