The peroxidase-antiperoxidase immunocytochemical technique has been used to examine the development of the ultrastructural features of somatostatin (SRIF)-immunoreactive neurons in the visual cortex of the rat between embryonic day 17 and postnatal day 32. In the adult, stained neurons are distributed in layers II through VI and characterized by an abundance of cytoplasm containing a plethora of organelles, most conspicuous of which are cisternae of granular endoplasmic reticulum organized in parallel arrays. In embryonic tissue, SRIF-positive cells are present in the subplate and in the border between the cortical plate and marginal zone. These cells possess scanty cytoplasm containing a few organelles; synapses onto immunoreactive perikarya and dendrites are evident at this stage. At birth and in early postnatal life, labelled cells are confined to the subplate region. Already at this age a number of cells display signs of ultrastructural features which characterize them in adult life. At the end of the first postnatal week, SRIF-immunoreactive neurons span a considerable spectrum of maturity. At one extreme are a few cells with little cytoplasm surrounding a large nucleus and at the other are the majority of labelled neurons showing abundant cytoplasm including prominent arrays of granular endoplasmic reticulum. Labelled cells first appear in the more superficial layers at the beginning of the second postnatal week and attain a distribution similar to that observed in adult animals at the end of this week. At this time their ultrastructural features closely resemble those of their adult counterparts, and differences in cytoplasmic maturity between superficial and deep labelled cells are not evident. This suggests that the SRIF-producing neurons in the superficial layers begin to express this peptide after they complete their migration and have acquired their morphological features. Maturation proceeds during the third postnatal week; at this stage most cells acquire their mature nuclear and cytoplasmic features and an adult complement of synapses. However, a number of SRIF-immunoreactive cells contain a particularly prominent accumulation of cytoplasmic organelles and appear hypertrophic.