The influence of early parental deprivation on the development of tyrosine hydroxylase- and 5-hydroxytryptamine-immunoreactive fiber innervation of subregions of the orbital prefrontal cortex (ventrolateral orbital, lateral orbital and agranular insular cortex) was quantitatively investigated in the precocious lagomorph Octodon degus. Forty-five-day-old degus from two groups were compared: 1) degus which were repeatedly separated from their parents during the first three postnatal weeks, and after weaning they were reared in social isolation; and 2) degus which were reared undisturbed in their families. Compared with the normal control animals the ventrolateral orbital prefrontal cortex and agranular insular cortex of the deprived animals displayed significantly increased density of tyrosine hydroxylase-immunoreactive fibers (up to 172% in the ventrolateral orbital prefrontal cortex and up to 143% in the agranular insular cortex). The lateral orbital prefrontal cortex showed increased 5-hydroxytryptamine-positive fiber densities (up to 118%). This altered balance between the serotonergic and dopaminergic cortical innervation in the orbital prefrontal cortex may reflect an anatomical and functional adaptation, which may be triggered by an altered activity of these transmitter systems during the phases of parental separation and social isolation.