In the present studies we have investigated if the increased expression of Gi alpha proteins reported earlier in heart and aorta from SHR (spontaneously hypertensive rats) is the cause or effect of hypertension. The SHRs at various ages of the development of blood pressure (3-5 days, 2 weeks, 4 weeks and 8 weeks) and their age-matched WKY were used for these studies. The expression of Gi alpha-2 and Gi alpha-3 (inhibitory guanine nucleotide regulatory protein and Gs alpha (stimulatory guanine nucleotide regulatory protein) at protein and mRNA level was determined by immunoblotting and Northern blotting technique using specific antibodies and cDNA probes. The SHR at early ages up to 2 weeks did not show any increase in blood pressure, however it started to go up from 4 weeks. The levels of Gi alpha 2 and Gi alpha 3 at protein and mRNA in heart from SHR were not different in 3-5 days old SHR as compared to WKY (Wistar-Kyoto rats), however, the expression of Gi alpha-2 and Gi alpha-3 protein and mRNA was significantly increased in 2 weeks and older SHR. The mRNA level of the catalytic subunit type V enzyme was significantly decreased in SHR 2 weeks and later ages as compared to their age-matched WKY. On the other hand, the expression of Gs alpha was not different in SHR as compared to WKY at all the ages studied. In addition, the oxotremorine and C-ANF4-23 (a ring deleted analog of atrial natriuretic factor) mediated inhibitions of adenylyl cyclase in hearts and aorta were also significantly enhanced in 2 weeks and older SHRs as compared to WKY rats, whereas, at younger age of SHR (3-5 days old), no change in the percent inhibition of adenylyl cyclase by C-ANF4-23 was observed and oxotremorine was unable to inhibit adenylyl cyclase activity. Furthermore, the basal enzyme activity and the stimulatory responses of isoproterenol, NECA (N-ethylcarboxamideadenosine), glucagon and forskolin on adenylyl cyclase were significantly decreased at all ages of SHR as compared to WKY. These results suggest that the increased expression of genes for Gi alpha-2 and Gi alpha-3, decreased expression of type V enzyme mRNA and decreased cAMP levels precedes the development of blood pressure and may participate in the pathogenesis of hypertension.