The development of alpha-1-fetoprotein (AFP)-producing cells was studied in rat liver by ultrastructural immunoperoxidase cytochemistry. AFP was detected in fetal and postnatal hepatocytes, including cell stages which ranged from the fetal to the adult cell-type. Irrespective of the stage of hepatocyte maturation, AFP was localized in the perinuclear space, the rough-surfaced endoplasmic reticulum (RER), and the Golgi apparatus. Non-membrane-bound ribosomes did not stain for AFP. Between Days 3 and 8 after birth, hepatocytes became comparable to those seen in adult liver. After Day 8, more and more unstained hepatocytes were observed, but AFP-staining cells with mature hepatocyte characteristics were still present. On Day 21, AFP was seen in short segments of the RER in few hepatocytes. After Day 28, AFP-staining cells were no longer detected. During postnatal liver maturation, AFP production was not due to the generation of morphologically distinct fetal-type hepatocytes.