The laminar distribution of theta (theta) field potentials in the entorhinal cortex (EC) was investigated in paralysed and locally anesthetized rats injected with physostigmine in order to induce theta rhythm. Electrode penetrations through the medial, intermediate and lateral subdivisions of the EC showed in all cases: 1. the presence of theta rhythm from layer VI to layer III approximately in phase with CA1 theta rhythm; 2. an amplitude minimum between the outer third of layer III and the inner half of layer I; and 3. a phase-reversed theta rhythm in layers II-I with an amplitude maximum in the outer half of layer I. Results indicate the existence of neuronal sources of theta rhythm in the EC.