The morphological characteristics of the basal dendritic fields of layer III pyramidal neurones were determined in visual areas in the occipital, parietal, and temporal lobes of adult marmoset monkeys by means of intracellular iontophoretic injection of Lucifer yellow. Neurones in the primary visual area (V1) had the least extensive and least complex (as determined by Sholl analysis) dendritic trees, followed by those in the second visual area (V2). There was a progressive increase in size and complexity of dendritic trees with rostral progression from V1 and V2, through the ''ventral stream,'' including the dorsolateral area (DL) and the caudal and rostral subdivisions of inferotemporal cortex (ITc and ITr, respectively). Neurones in areas of the dorsal stream, including the dorsomedial (DM), dorsoanterior (DA), middle temporal (MT), and posterior parietal (PP) areas, were similar in size and complexity but were larger and more complex than those in V1 and V2. Neurones in V1 had the lowest spine density, whereas neurones in V2, DM, DA, and PP had similar spine densities. Neurones in MT and inferotemporal cortex had relatively high spine densities, with those in ITr having the highest spine density of all neurones studied. Calculations based on the size, number of branches, and spine densities revealed that layer III pyramidal neurones in ITr have 7.4 times more spines on their basal dendritic fields than those in V1. The differences in the extent of, and the number of spines in, the basal dendritic fields of layer III pyramidal neurones in the different visual areas suggest differences in the ability of neurones to integrate excitatory and inhibitory inputs. The differences in neuronal morphology between visual areas, and the consistency in these differences across New World and Old World monkey species, suggest that they reflect fundamental organisational principles in primate visual cortical structure.