Endocytosis of gastrin was studied in a number of gastrin-receptor-expressing cell lines by confocal laser scanning microscopy (CLSM) with the aid of a biologically active fluorescent derivative, rhodamine green heptagastrin. Rapid clustering (within 4-7 min) and internalization of fluorescent ligand upon binding at room temperature and 37 degrees C were observed in the rat pancreatic acinar carcinoma cell line AR42J, human gastric carcinomas AGS-P and SIIA, human colon carcinomas HCT116 and HT29, and in NIH/3T3 cells transfected with human and rat gastrin/cholecystokinin-B receptor cDNA. Internalization was inhibited by hypertonic medium. Fluorescent heptagastrin and transferrin colocalized in the same endocytic vesicles at different stages of internalization suggesting that endocytosis occurred predominantly through a clathrin-dependent mechanism. At 37 degrees C partial colocalization with the lysosomal marker neutral red was detected by CLSM, implying that internalized gastrin accumulated in the lysosomes. Immunoelectron microscopy studies with antibodies against gastrin revealed the presence of the internalized hormone in multivesicular vesicles and endosomes. Almost no hormone was detected in lysosomes with the antibodies to gastrin, suggesting that the degradation of the peptide is rapid in those vesicles. Continuous accumulation of fluorescent label was observed by CLSM in the presence of the protein synthesis inhibitor cycloheximide, suggesting that the gastrin receptor is recycled back to the cell membrane after hormone delivery to intracellular compartments. An estimated average recycling time for the receptor molecules was 1 h in NIH/3T3 cells.