This ultrastructural study compared the endocytosis of a peptide hormone, ferritin-labeled insulin (Fm-I) or gold-labeled insulin (Au-I), and a non-hormonal ligand, gold-labeled alpha-2-macroglobulin-methylamine (Au-alpha 2MGMA), by rat adipocytes. Quantitative analysis of the cell surface showed that coated pits occupied 0.4% of the adipocyte surface. This was one fifth to one tenth of that which has been reported on fibroblasts and hepatocytes, cell types in which receptor-mediated endocytosis has been extensively studied. In contrast, uncoated micropinocytotic invaginations were quite numerous and occupied 13.1% of the adipocyte cell surface. The frequency of micropinocytotic invaginations, 13.8 per micron 2 of plasma membrane, was 7-12 times greater than has been reported on fibroblasts. Therefore, the ultrastructure of the endocytic apparatus on rat adipocytes was different from more commonly studied cell types. At 4 degrees C, Au-alpha 2MGMA concentrated within coated pits to a density that was 52 times greater than that on the uncoated plasma membrane. Au-alpha 2MGMA was excluded from micropinocytotic invaginations by more than 93%; this exclusion was unrelated to the size of the Au-alpha 2MGMA particle. In contrast, at 4 degrees C, Fm-I did not concentrate within coated pits and occupied micropinocytotic invaginations in a random manner. At 37 degrees C, coated pits accounted for all of the endocytosis of Au-alpha 2MGMA, proving that these structures were functional despite their atypically low density. In contrast, greater than 99% of the endocytosis of Fm-I or Au-I occurred through micropinocytotic invaginations. These results demonstrated for the first time by a comparative, quantitative, ultrastructural method that insulin and Au-alpha 2MGMA undergo endocytosis by dissimilar mechanisms on rat adipocytes. Dissimilarities in the endocytosis of insulin and Au-alpha 2MGMA may be related to the different biological roles of these two molecules.