The movements of the molecular components of the luminal plasma membrane during exocytotic secretion in parotid acinar cells were examined. For immunocytochemical study, we used an antiserum of dipeptidyl peptidase IV as a marker for the components of the luminal plasma membrane of acinar cells. In unstimulated acinar cells, dipeptidyl peptidase IV immunoreactivity is restricted to the luminal plasma membrane. However, after secretion was stimulated with a beta-adrenergic agonist, isoproterenol, immunostaining became detectable on the membrane of discharged granules. Freeze-fracture images showed that the density of intra-membrane particles on the P-fracture leaflets of discharged granule membranes is much higher than that of undischarged granule membranes during secretion. These results suggest that in parotid acinar cells of the rat, the components of the luminal plasma membrane move laterally, during secretion, to the membranes of discharged granules.