We investigated the afferents and intracortical synaptic organization of the anterior cingulate cortex (ACC) during noxious electrical stimulation. Extracellular field potentials were recorded simultaneously from 16 electrodes spanning all layers of the ACC in male Sprague–Dawley rats anesthetized by halothane inhalation. Laminar specific transmembrane currents were calculated with the current source density analysis method. Two major groups of evoked sink currents were identified: an early group (latency 54.04±2.12 s; 0.63±0.07 mV/mm²)in layers V–VI and a more intense late group (latency 80.07±4.85 s; 2.16±0.22 mV/mm²) in layer II/III and layer V. Multiunit ctivities were evoked mainly in layer V and deep layer II/III with atencies similar to that of the early and late sink groups. The evoked EPSP latencies of pyramidal neurons in layers II/III and V related closely with the sink currents. The sink currents were inhibited by intracortical injection of CNQX (1 mM, 1 l), a glutaminergic receptor antagonist, and enhanced by intraperitoneal (5 mg/kg) and intracortical (10 g/ l, 1l) injection of morphine, a opioid receptor agonist. Paired-pulse depression was observed with interpulse intervals of 50 to 1,000 ms. High-frequency stimulation (100Hz, 11 pulses) enhanced evoked esponses in the ACC and evoked edial thalamic (MT) unit activities. MT lesions blocked voked esponses in the ACC. Our results demonstrated that two distinct ynaptic circuits in the ACC were activated by noxious stimuli and hat the MT is the major thalamic relay that transmits nociceptive information to the ACC.