Pro-vasopressin-neurophysin mRNA (AVP mRNA) was assayed in the pituitary gland of rats from different strains, using a specific cRNA probe corresponding to exon C of the vasopressin gene. Northern blot analysis revealed the presence of an about 650 base pair (bp) AVP mRNA in the neurolobe of all strains. No signal was found in the anterior or intermediate lobes. Partial nucleotide sequence analysis suggested identity with the genuine AVP mRNA. Osmotic stimulation with 2% saline or water deprivation increased AVP mRNA levels in a fashion that paralleled changes seen in the hypothalamus. The origin of the AVP mRNA in the neurolobe is uncertain but is likely to be the pituicytes. Transport of the mRNA into the nerve endings of hypothalamic supraoptic and paraventricular neurons is an alternative possibility.