Immunoactive antidiuretic hormone (ADH) was measured by radioimmunoassay in the plasma, lung tumours and metastatic tumours of nineteen patients with bronchogenic carcinoma. Ten patients had hyponatraemia and carcinoma of the small oat cell type. Plasma ADH measured in nine of these patients ranged from 11--270 pg/ml and was elevated above the normal range (4.6--6.2 pg/ml) in all subjects. ADH-immunoreactive material was detectable in all primary lung tumours (range 9--1080 pg/mg wet weight, n = 7) and metastases (range 5--63 pg/mg wet weight, n = 9) obtained from the hyponatraemic patients. A statistical relationship existed between plasma and tumour ADH concentration in six patients where both measurements were performed. Three patients had small cell carcinomas (two oat cell and one anaplastic) without overt hyponatraemia. ADH-like material was detectable in the lung tumours (18 and 1.1 pg/mg wet weight) and liver metastases (4 and 1.0 pg/mg wet weight) of two patients but not in the third. Four of the remaining patients had squamous cell carcinomas and two had adenocarcinomas. None had hyponatraemia. ADH-like material was undetectable in all lung tumours, metastatic tumours and uninvolved tissue from these patients. ADH extracted from the pituitaries of four patients ranged from 6400--13200 pg/mg wet weight. ADH immunoreactive extracts of six lung tumours and nine metastases (all oat cell) showed the same pattern on elution from a Sephadex G-25 column. A large peak, which made up 65% of the total activity, was eluted in the same position as synthetic arginine vasopressin and contained comparable amounts of immunoreactive and bioactive ADH. Two smaller peaks (8 and 27% of total activity) were eluted in positions of higher molecular weight and contained more immunoreactive than bioactive ADH. In contrast, three of four pituitary extracts showed only a single peak which eluted in the same position as marker vasopressin.