Proopiomelanocortin (POMC), the common precursor to beta-endorphin and alpha-melanocyte-stimulating hormone synthesized in rat intermediate lobe cells, exhibits both charge and size heterogeneity on two-dimensional gels. Pulse-labeling and pulse-chase studies revealed that this heterogeneity is due to co- and post-translational modifications of a single common polypeptide. Short 5-min-pulse incubation with [3H]phenylalanine allowed the preferential labeling of two major forms characterized by an identical isoelectric point (8.2), but slightly different apparent molecular weights (MW = 34,000 and 36,000). These peptides could be labeled with [3H]mannose and the analysis of their tryptic fragments by high-pressure liquid chromatography revealed that they correspond to polypeptides bearing one or two N-linked carbohydrate side chains. Accumulation of more acidic forms was observed during subsequent chase incubations in the absence of phenylalanine. These acidic forms were shown to incorporate sulfate and (or) phosphate groups. Sulfation and phosphorylation occurred on POMC within 5 min after its synthesis and were concomitant with the processing of the N-linked carbohydrates from the high mannose to the complex structure. Finally, partial digestion of the phosphorylated and nonphosphorylated analogs of POMC with either Staphylococcus aureus (V8 strain) protease or chymotrypsin suggests that the presence of a phosphate group may alter POMC sensitivity to exogenously added proteases.