We have recently shown that ANF-R2 receptors are coupled to the adenylyl cyclase/cAMP system through inhibitory guanine nucleotide regulatory protein (Gi). The present studies were undertaken to investigate the regulation of ANF-R2 receptor-mediated inhibition of adenylyl cyclase in spontaneously hypertensive rats (SHR) which have been reported to have high plasma ANF levels. ANF99-126 inhibited adenylyl cyclase activity in a concentration dependent manner in both aorta and heart sarcolemma from SHR and age-matched Wistar-Kyoto (WKY) rats, however, the extent of inhibition was greater in SHR as compared to WKY. On the other hand, ANF99-126 also inhibited the adenylyl cyclase activity in rat platelets from WKY rats in a concentration dependent manner, however, the inhibition in SHR was completely attenuated. In addition GTP gamma S stimulated adenylyl cyclase activity by about 600% in aorta from WKY rats and about 300% in SHR. On the other hand, the GTP gamma S-stimulated adenylyl cyclase activity was higher in platelets from SHR as compared to WKY. The observed difference may be attributed to the differential alterations in ANF-R2 receptor number or postreceptor events or both. Nonetheless, these results indicate that ANF-R2 receptors in platelets from SHR are differentially regulated than those in heart and aorta.