Rabbit retinas were incubated in medium containing 500 microCi of [(3)H]leucine for 3 min, and transferred to medium without isotope for another 7, 17, 37, 57, and 117 min. Retinal pieces were fixed in paraformaldehyde and osmium tetroxide and embedded in Epon. Thin sections were autoradiographed with Ilford L4 emulsion, and a quantitative study of silver grain distribution per Müller cell portion, and per Müller cell organelle, was carried out. Grain density per unit area was high over the middle cell portion at each incubation interval. Silver grains were numerous over background cytoplasm (which comprised free ribosomes) but their percentage was constant at all times and their relative concentration low. Silver grains were numerous and highly concentrated, at pulse incubation, over the rough endoplasmic reticulum (RER) and then decreased sharply, but this decline coincided with an increase over the Golgi complex, peaking at 20 min. Another peak appeared over the cell periphery at 60 min. These findings suggest the simultaneous synthesis of two types of proteins in Müller cells; structural proteins in background cytoplasm and proteins of secretory type in the RER.