The retinal pigment epithelium (RPE) lying along the vertical meridian of the eyes of wild-type (+/+) and pearl mutant (pe/pe) mice was examined with electron microscopy and microspectrophotometry in order to compare differences in melanosome location, size, numerical density, volume density, and melanin concentration. In +/+ mice the fraction of melanosomes that lie in the apical processes was greater in the superior than in the inferior retina. Also, the numerical density and volume density of soma melanosomes were lower in the superior retina. The soma melanosomes were also located closer to the basal membrane and had a larger average diameter in the inferior retina. No significant differences in melanosome location were observed between light-adapted and dark-adapted retinas. In pe/pe mice the fraction of melanosomes in the apical processes was very low in both inferior and superior retinas. The numerical density and the volume density of soma melanosomes of pe/pe were significantly less than those of +/+. The soma melanosomes of pe/pe mice lie closer to the basal membrane than those of +/+, and they had a larger mean diameter than those of +/+. Electron micrographs depicted some melanosomes of pe/pe with irregular profiles and abnormal melanin deposition at their periphery. Microspectrophotometric measurements of individual melanosomes in semithin sections showed that maximal specific absorption and lambda max were nearly equal for both genotypes and both superior/inferior locations. These findings show that intramelanosomal melanin concentrations is normal for pe/pe. The basal membrane of the RPE of pe/pe was infolded over only 80% of the area covered by the basal surface, whereas the basal membrane in +/+ was infolded over more than 98% of the area covered. The boundary density of RPE basal infoldings was also lower in pe/pe than in +/+. Furthermore, some areas of the basal lamina of pe/pe RPE had a periodic, rather than an amorphous, structure. These findings suggest that the reduced retinal sensitivity found in intact pe/pe animals, but not in superfused, isolated pe/pe retinas, may result from impaired transport of a diffusible substance by the basal membrane of the RPE.