Studies performed on 5 cancer patients (3 bronchogenie carcinomas, 1 pancreatic carcinoma, 1 female choriocarcinoma) resulted in evidence that (1) some malignant tumors may contain a small amount of antidiuretic activity identical or very similar to posterior pituitary antidiuretic hormone (ADH) even in the absence of the syndrome of inappropriate ADH secretion; (2) the zone of active tumor growth contained most (50–100%) of the antidiuretic activity; (3) para-endocrine-active tumor cells can release ADH into Krebs-Ringer incubation solution, and they may be capable of synthesizing a substance identical or very similar to posterior pituitary ADH; (4) ADH levels measured in blood plasma and urine reflect the overall ADH production rate in tumor tissue.