1. In order to explore the mechanisms responsible for the hypernoradrenergic innervation of the vasculature in the spontaneously hypertensive rat (SHR) the tissue content of nerve growth factor messenger ribonucleic acid (NGFmRNA) was examined. 2. The concentration of NGFmRNA was markedly elevated in mesenteric veins obtained from SHR when compared with the contents of NGFmRNA in veins from Wistar Kyoto rats (WKY). 3. The NGFmRNA content of kidneys was greater in SHR when compared with the levels present in WKY rats for 10- and 43-day-old animals. 4. In contrast to the pattern observed for veins and kidneys, the NGFmRNA content of SHR hearts was smaller than those present in hearts from WKY rats for 2, 10 and 43-day-old animals. 5. The results demonstrate that tissues with enhanced innervation (the kidney and mesenteric vasculature) in SHR are associated with an enhanced expression of NGFmRNA. In contrast, the heart, which does not display an enhanced sympathetic innervation in the SHR, does not have an increased expression of NGFmRNA. 6. It is suggested that in the SHR there is a tight relationship between hypernoradrenergic innervation in the vasculature and gene expression for NGFmRNA.