Peripherin is mainly expressed in peripheral neurones and in CNS neurones which extend axons into peripheral nerves. However, this intermediate filament protein has also been detected in a few other neurones entirely located within the CNS. The present study focuses on the adult rat neocortex. Peripherin immunoreactive (P+) neuronal somata and their neuritic extensions were observed in cortical layers II, III, V and VI, while a few P+ nerve fibres could be seen in layer I. All the P+ neurones could be selectively stained using reduced nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) histochemistry, a typical feature of aspiny neurones. Some of the P+ neurones could also be immunostained with an antibody raised against nitric oxide synthase (NOS). These results provide evidence that peripherin is present in a discrete population of aspiny interneurones of the adult rat neocortex. The functional significance of the co-expression of peripherin and NOS needs further investigation.