Regional and laminar distribution patterns of neurofilament proteins in the dorsolateral premotor cortex (PMd) were studied with monoclonal antibody SMI-32 in five adult macaque monkeys and compared with the cytoarchitectonical features of the PMd. Our goal was to reveal whether the increasing functional diversity of the PMd which electrophysiological studies have unravelled over the last years is reflected on a structural level by differences in the neurochemical phenotype. Differences in size, shape and packing density of immunopositive layer III and V pyramidal cells define areas much more clearly than do differences in cytoarchitecture. The PMd can be subdivided into a rostral and a caudal part at a level slightly anterior to the genu of the arcuate sulcus. The extent of these two areas matches the two cytoarchitectonically defined areas F7 and F2, respectively. Within area F2, differences in layer V immunoreactive neurons define a dorsal (F2d) and a ventral (F2v) region. The border between areas F2d and F2v lies at the superior precentral dimple and cannot be detected cytoarchitectonically in Nissl-stained sections. Neurofilament proteins are involved in the stabilization of the cytoskeleton of the axon and have been correlated with axonal size and conduction velocity of nerve fibres. This regional variability in the neurochemical phenotype of layer V within the caudal PMd may reflect a differential organization of the descending output from this part of the premotor cortex. It might also be related to differences in the motor control of voluntary arm and leg movements.