We developed a method, termed an H-blot, by which the poly (A) tract of any specific mRNA may be detected by RNA filter hybridization after its removal from the body of the mRNA by a RNase H-catalyzed endonucleolytic cleavage in the 3′ untranslated region. Using this method, we studied the modulation of the length of the poly (A) tract of rat vasopressin mRNA in vivo during changes in the levels of this mRNA resulting from a physiologic stimulus, osmotic stress. The poly (A) tract of hypothalamic vasopressin mRNA in hydrated rats was, quite remarkably,~ 250 nucleotides in length, in contrast to that of somatostatin mRNA, which was~ 30 nucleotides long. Vasopressin mRNA poly (A) tail length increased progressively from~ 250 to~ 400 nucleotides with the application of the hyperosmotic stimulus and declined to base line after its removal; somatostatin mRNA poly (A) tail length did not change during osmotic stress. The poly (A) tract length of total hypothalamic mRNA was between 35 and 140 nucleotides and also did not change with osmotic stress. Modulation of poly (A) tract length of specific mRNAs during stimulation of gene expression may provide an additional level of genetic regulation.