In the adult nervous system, vasoactive intestinal polypeptide acts as a neurotransmitter or neuromodulator, and during development, it may also act as a neurotrophic factor. In the adult mammalian retina, this peptide is contained in a population of wide-field amacrine cells. Using in situ hybridization histochemistry, we examined the distribution and developmental expression of vasoactive intestinal polypeptide/peptide histidine isoleucine messenger RNA in the rat retina. Retinas collected from birth to adulthood were hybridized with an RNA probe as whole mounts, and then cut either perpendicular or parallel to the vitreal surface. Adult retinas were used in double labeling experiments for the visualization of both the hybridization signal and vasoactive intestinal polypeptide immunoreactivity in the same tissue section. In adult retinas, vasoactive intestinal polypeptide/peptide histidine isoleucine messenger RNA is localized to amacrine cells positioned in the proximal inner nuclear layer, and rarely to displaced amacrine cells in the inner plexiform layer and ganglion cell layer. The neurons expressing this messenger RNA are sparsely distributed, with a non-random distribution and densities of about 190 cells/mm2. An estimate of their total number gives about 12,350 cells/retina. The double labeling experiments showed that the hybridization signal is specifically confined to neurons displaying vasoactive intestinal polypeptide immunoreactivity. Vasoactive intestinal polypeptide/peptide histidine isoleucine messenger RNA is first detected at postnatal day 5 in cells located in the proximal part of the neuroblastic layer. A greater number of these neurons is present in the inner nuclear layer at postnatal day 10, and a few labeled neurons are also detected in the inner plexiform layer and in the ganglion cell layer. At this time, vasoactive intestinal polypeptide/peptide histidine isoleucine messenger RNA-containing amacrines in the inner nuclear layer are non-randomly distributed on the retinal surface, as in adult retinas. At postnatal day 15 (eye opening), there is a peak in both the density and the estimated number of labeled neurons, and their pattern of distribution in the retinal layers is similar to that in the adult. The present study shows that in the adult rat retina vasoactive intestinal polypeptide and peptide histidine isoleucine are synthesized in a sparsely distributed amacrine cell population, extending previous immunohistochemical findings. The appearance of vasoactive intestinal polypeptide peptide histidine isoleucine messenger RNA during the first postnatal week is consistent with the reported appearance of other transmitter-identified amacrine cell populations.(ABSTRACT TRUNCATED AT 400 WORDS)