The adenylate-cyclase activity contained in the successive segments of the loop of Henle was measured using single tubular pieces isolated by microdissection. Vasopressin (10 (-6) M) had no significant effect on either the pars recta or the thin descending limb; whereas, it stimulated the enzyme contained in the thin ascending limb (8.9 fold as a mean value) and that contained in the medullary portion of the large ascending limb (14.7 fold).