We have studied the anatomical localization, chromatographic properties, and development of somatostatin-like immunoreactivity (SLI) in the guinea pig retina. The majority of guinea pig retinal SLI in the adult and fetus eluted similarly to somatostatin-28 by gel filtration and high pressure liquid chromatography. This represents a higher ratio of somatostatin-28-like material to total SLI than had been observed in retinal extracts from most other animal species. Somatostatin-like-immunoreactive cells and fibers were localized using two antisera in cryostat-sectioned and flat mounted retinae. Distribution of cells and fibers differed uniquely from that in other species previously reported. Reactive perikarya were located only in the far peripheral region: in the innermost layer of the inner nuclear layer (INL), in the inner plexiform layer (IPL), and in the ganglion cell layer. Reactive fibers were prominent in the IPL and nerve fiber layer (NFL) in both the peripheral and central retina. Less frequently, processes were observed between the NFL and IPL, between the IPL and INL, and, rarely, in the outer plexiform layer and outer nuclear layer. Small numbers of reactive fibers were found in the optic nerve and disc. These observations suggest that processes of intrinsic (amacrine or associational ganglion cells) and projection neurons (true ganglion cells or efferent fibers) containing SLI are intermingled in the guinea pig retina. SLI, quantified by radioimmunoassay, was present in the developing retina as early as the 6th week of gestation (full term is 10 weeks). Immunohistochemically detected somatostatin-like-immunoreactive elements were seen first at 2 weeks before birth, coincident with the onset of the period of most rapid increase in levels of assayed SLI. Somatostatin-like-immunoreactive peptides reached two-thirds of adult levels by birth.