Freeze-fracture electron microscopy had previously revealed antidiuretic hormone-induced aggregates of intramembranous particles in amphibian urinary bladder. To investigate the effects of antidiuretic hormone (ADH) in another ADH-sensitive epithelium, namely, mammalian renal collecting ducts, freeze-fracture studies were carried out in Brattleboro homozygous rats. Collecting duct luminal membranes of ADH-treated homozygotes showed intramembranous particle clusters (117 ± 17/100 μm2) that were loosely packed and that occurred on both exoplasmic (E) and protoplasmic (P) faces. Untreated, control homozygous rats had significantly less (3 ± 1/100 μm2) clusters. Changes similar to those seen in ADH-treated rats were observed in water-deprived Wistar rats. The clustered particles differed from those seen in ADH-treated amphibian urinary bladder in that the latter occurred only on the P face and were more densely packed. Nevertheless, our observations suggest a common membrane effect for ADH action that may apply in mammals and amphibia alike. freeze-fracture; Brattleboro homozygous rats; membrane particle clusters Submitted on March 6, 1978 Accepted on July 14, 1978