cGMP is synthesized via nitric oxide- or natriuretic peptide-stimulated guanylyl cyclases and exhibits pleiotropic regulatory functions also in the kidney. Hence, the integration of cGMP signaling via cGMP-dependent protein kinases (cGK) might play a critical role for renal physiology. Both isozymes were detected in arterioles, mesangium and within the cortical interstitium. In contrast to cGKIa, the ß isoform was not detected in the juxtaglomerular apparatus and medullary fibroblasts. Here, we examined the function of cGKI in the interstitium emphasizing a functional differentiation of both isoforms. The interstitium exists mainly of fibroblasts playing a prominent role in the interstitial fibrosis. Accordingly, cGKI could also be involved in this pathophysiological process. Therefore, we studied whether cGKI influences renal fibrosis by application of cGMP increasing YC1 or ISDN and by using mutant mice. The fibrosis was induced by unilateral ureter obstruction (UUO). Administration of ISDN showed antifibrotic effects in wt mice but not in mutant mice. Moreover, mRNA- and protein expression of cGKIb was significantly less influenced by fibrosis than cGKIa. Accordingly, our results indicate that cGMP acts via cGKI as an important suppressor of kidney fibrosis.