The water permeability of collecting ducts is greatly increased by the antidiuretic hormone, vasopressin (VP). Freeze-fracture studies were carried out to test if this permeability increase is associated with the appearance of intramembrane particle (IMP) aggregates and whether increased doses of VP lead to an increase in the number and size of particle aggregates in the luminal membrane of principal cells in the isolated cortical collecting duct. Unstimulated cells expressed 17 +/- 6.5 particle aggregates per 100 microns 2. Stimulation with VP at concentrations of 20 or 200 microU/ml increased the number of particle aggregates significantly to 129 +/- 15.8 and 324 +/- 45.8, respectively. The size of the particle aggregates increased from 0.0012 microns 2 under control conditions to 0.025 microns 2 at 20 microU/ml VP and to 0.063 microns 2 at 200 microU/ml VP. In addition, the total area occupied by the IMP increased from 0.02 microns 2/100 microns 2 (controls) to 3.17% and 20.38% (after 20 and 200 microU ADH/ml, respectively). Particle aggregates were also observed in the luminal plasma membrane of isolated collecting ducts fixed immediately after dissection, resembling the in vivo status. These results demonstrate that a dose-dependent relationship exists between the concentration of the applied VP and the number of particle aggregates, as well as the size of the aggregates. Cytoplasmic tubular vesicles in fusion with the apical membrane were observed.