The electrophysiological and morphological features of neurons in the rat rostral claustrum were examined using intracellular recording and staining methods in vivo. A total of 31 neurons were analysed electrophysiologically, and 21 of these were stained well with intracellular biocytin injection. The following electrophysiological properties were analysed by intracellular current injection: firing properties, the shape of single action potentials and input resistance. The firing patterns of the claustral neurons seemed to be similar to those of regular spiking cells in the cerebral cortex. They had action potentials with a maximum rate of rise much higher than that of fall, and showed spike-frequency adaptation during long depolarizing pulses. The morphological analysis demonstrated that the claustral neurons were of various types: the somata were polygonal, triangular, ovoid, round, or fusiform, sometimes with a stout dendrite. Such a dendrite extended toward the superficial layers in the more rostral orbital cortex, and was revealed to be a distorted apical dendrite by a three-dimensional computer-aided system. The embryological origin of the claustrum has been a matter of controversy: two main hypotheses of cortical germinal origin and ganglionic eminence origin. Considering the firing patterns and morphological features, the present findings suggest that the neurons in the rostral claustrum share some physiological characteristics with cortical neurons in rats.