The early expression of neurotransmitters and receptors in the developing brain has brought attention to their potential contribution in modulating neuronal developmental processes. Monoamines are among the first neurotransmitter systems to develop during embryogenesis. Depletion of neocortical serotonin or catecholamine afferents with selective neurotoxins resulted in a permanent alteration of the dendritic arborization of calretinin-containing interneurons, and a transient delay of parvalbumin and calbindin expression in a number of cortical neurones during the second postnatal week. The expression pattern of other developmentally regulated proteins, such as two subunits of the GABAAreceptor, was not altered. Depletion of serotonin, and in part catecholamines, appeared to perturb several developmental processes of the cerebral cortex which would interfere with both its maturation and adult circuitry.