Concentrations of vasopressin (VP) precursor and oxytocin (OT) precursor mRNA were measured in magnocellular cell groups of the rat hypothalamus by newly developed solution hybridization assays. The assays employed single-stranded 35S-labeled VP-specific and OT-specific DNA probes that were prepared by primer extension on recombinant M13 DNA templates. Solution hybridization assays were standardized by known amounts of cloned DNA. The detection limit was less than 1 pg DNA equivalent of the respective mRNA. In total RNA preparations of microdissected supraoptic nucleus (SON) mean (+/- SEM) basal levels of 1.37 +/- 0.18 pg VP mRNA and 1.95 +/- 0.14 pg OT mRNA were measured. RNA of the microdissected paraventricular nucleus (PVN) contained 0.35 +/- 0.02 pg VP mRNA and 1.77 +/- 0.15 pg OT mRNA. Elevation of plasma osmolality induced by drinking of 2% saline for 25 days resulted in a 1.85-fold increase in VP mRNA levels of the SON and a 1.6-fold increase in VP mRNA levels of the PVN. The solution hybridization assays are suitable tools to study the regulation of VP and OT mRNAs in magnocellular neurons of the brain.