Nitric oxide (NO) is a ubiquitous gaseous neurotransmitter that has been ascribed to a large number of physiological roles in sensory neurons. It is produced by the enzyme nitric oxide synthase (NOS). To identify the NOS-containing structures of rat trigeminal primary afferent neurons, located in the trigeminal ganglion (TrG) and mesencephalic trigeminal nucleus (MTN), histochemistry to its selective marker nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) was applied in this study. In the TrG approximately half of the neuronal population was NADPH-d reactive. Strongly positive were neurons mainly of small-to-medium size. Neuronal profiles of large diameter were less intensely stained. In addition, NADPH-d-positive nerve fibers were dispersed throughout the ganglion. Nitrergic neurons were located in the caudal part and mesencephalic-pontine junction of the MTN. Most of them were large-sized pseudounipolar cells. In a more rostral aspect, the reactive psedounipolar MTN profiles gradually decreased in number and intensity of staining. There, only a fine meshwork of stained thin fibers and perisomatic terminal arborizations, and also some isolated perikarya of NADPH-d stained multipolar MTN neurons, were observed. The predominant NADPH-d localization in smaller in size TrG neurons, which are considered nociceptive, suggests that NO may play a role in the pain transmission in the rat trigeminal afferent pathways. In addition, the wide distribution of NADPH-d activity in large pseudounipolar and certain multipolar MTN neurons provides substantial evidence that NO may also participate in mediating proprioceptive information from the orofacial region. The differential expression patterns of nitrergic fibers in the TrG and MTN suggest that trigeminal sensory information processing is controlled by nitrergic input through different mechanisms.