The rat kidney and brain are major target organs for vasopressin (VP). A procedure was developed for immunocytochemical staining of VP and its binding sites in the kidney. This procedure involved preincubation of kidney sections with the ligand, followed by immunocytochemical detection of VP. The staining in renal tubules from Wistar rats was enhanced by preincubation of tissue sections with increasing concentrations of VP (6-6000 nmol/l). Staining was present in the epithelium of distal convolutions and collecting ducts (medullary and cortical portions) and more pronounced in the apical zone of the tubular epithelium. With high concentrations of VP in the preincubation, staining was also obtained in the thick ascending limb of the loop of Henle. There was no staining under any circumstances in proximal tubules. In the kidney of the Brattleboro rat homozygous for hypothalamic diabetes insipidus (DI) which congenitally lacks VP but responds to the peptide, exactly the same staining pattern was observed after preincubation with VP, but the maximal staining was less intense. The VP binding to the DI rat kidney, after 2 weeks treatment with VP (using Accurel implants), reached levels seen in the Wistar kidney after in-vitro preincubation with high doses of VP.