Summary. Unit activity recorded from the cingulate cortex during theta rhythm shows periodic trains of spikes which are phase-locked to the local theta field potential waves. These cortical theta units were also shown to be correlated with hippocampal theta units. These findings, along with the fact that theta field potentials show a phase reversal within the cingulate cortex, lead to the conclusion that this cortical area is a source of theta activity.Key words: Theta rhythm -Spike trains -Field potential -Cingulate cortex -Hippocampus Theta field potentials, or rhythmic slow activity (RSA) in the frequency band 4-10 Hz in the rat, recorded in parts of the cortex overlying the hippocampal complex, have been noted by several authors. Although without specifying the cortical areas from which it was recorded, Artemenko (1972), Winson (1974), Bland and Whishaw (1976), and Gerbrandt et al. (1978 described cortical theta to be isomorphic with theta in the neighboring part of the hippocampus, the amplitude decreasing toward the cortical surface. Cortical theta would thus be accounted for in terms of volume conduction theory, since a stationary neuronal source in a purely resistive medium causes a field potential, the amplitude of which decreases with increasing distance from this source without phase shift (Holsheimer and Feenstra 1977).However, Petsche and Stumpf (1960) found a "loose phase coupling" between cortical and hippocampal theta field potentials in the rabbit; the theta waves in these two regions were never more than a fraction of a period apart. During arousal they found regular theta rhythm in the hippocampus, but irregular theta activity in the cortex of the awake rabbit. Winson (1974) reported that both intermittent theta or absence of theta in the cortex could be found during regular hippocampal theta rhythm in the awake rat. These findings suggest that theta field potentials in parts of the cortex overlying the hippocampal complex may not be considered solely the result of volume conduction from a hippocampal source, i.e., there is a possibility that hippocampal and cortical theta have different neuronal sources.We made a quantitative analysis of the relationship between theta field potentials in the laminae of the dorsal hippocampus and overlying cortex of the urethane anesthetized rat, which indeed revealed phase differences of approximately 180 deg not only in the hippocampus but also in the overlying cortex Holsheimer et al. 1979). These theta phase reversals, on tracks normal to the hippocampal and to the cortical surface (laminar profiles), were considered as indicating that neuronal sources of theta activity exist in each of these structures; each source causing a dipole-like potential field. The cortical areas concerned were those parts lying near the midline, namely the cingulate cortex (Fig. 1). The corresponding neuronal source might be the population of large pyramidal cells having their somata in layer V, since the theta phase reversal was found in this layer. We also found that the coher...