Experiments were performed to get insight into the role of arginine vasopressin (AVP) receptor V_1a regulation with age, i.e. during development and maintenance of high blood pressure. Previous studies have shown an increased gene expression and renal vascular response to AVP and in young spontaneously hypertensive rats (SHR). The age regulation of the V1a receptor was examined in preglomerular vessels from 5, 10, 20 and 70 weeks old SHR using the normotensive Wistar Kyoto rats (WKY) as controls. Real time PCR and ligand binding were used for analysis of receptor expression and change in cytosolic calcium concentration during stimulation of isolated preglomerular vessels with AVP was studied. Studies showed an increase of the V1a receptor protein and mRNA from 5 and 10 weeks old SHR compared to vessels from 20 and 70 weeks old SHR. In 5 weeks old SHR receptor density was 84 ± 13, and 38 ± 11 fmol/mg protein in 70 weeks old SHR (p<0.05).mRNA in the 5 and 70 weeks old SHR was 15854 ± 629 and 3181±224 V1a mRNA/10₈ 18S ribosomal RNA respectively (p<0.001). Values from WKY at all ages were similar to 20 and 70 weeks old SHR. During stimulation with AVP, change in cytosolic calcium in vessels from 5 week old SHR increased 234 ± 59 nM while the increase was 89 ± 9 nM in 70 weeks old SHR (p = 0.03). These results indicate that the V_1a receptor is increased at protein and mRNA level during development of hypertension in SHR, but is normalized when hypertension is established.