The nitric oxide/cyclic guanosine phosphate pathway plays a pivotal role in vasodilatation and as such also in penile erection. Because of its central role in this molecular pathway, sGC represents a very attractive and promising new target for the development of new treatments for hypertension and/or erectile dysfunction. The cloning of sGC from various species has revealed that the protein is a heterodimer composed of a larger a and a smaller b subunit, which are both required for catalysis. Although two a and two b isoforms have been characterised so far, only the sGCa₁b₁ and the sGCa₂b₁ isoforms have been shown to occur in vivo. To establish the functional role of sGC and its different isoforms in the mechanism of vasodilatation and penile erection we performed in vivo studies on b₁ His 105 Phe transgenic mice (sGCb₁^ki/ki mice) and their littermates. Different agents were injected either intravenously or intracavernosally and the changes in mean arterial pressure (MAP) and intracavernosal pressure (ICP) were recorded in the anesthetized mice. Intravenous and intracavernosal injection of exogenous NO (SNP – Spermine/NO) resulted in a decrease of MAP and an increase in ICP respectively in the wild-type controle mice. These responses are however completely abolished in sGCb₁^ki/ki mice. Intravenous administration of L-NAME which induced an increase in MAP in sGCb₁^+/+ mice, had no effect when injected in sGCb₁^ki/ki mice. Stimulation of the nervus cavernosus induced frequency-dependent increases in ICP in sGCb₁^+/+ mice but again no response could be observed in sGCb₁^ki/ki mice. Responses to the sGC-independent agents forskolin and 8-pCPT-cGMP which were injected intracavernosally did not differ between the sGCb₁^ki/ki mice and the sGCb₁^+/+ mice. These studies indicate that the NO-dependent vasodilatation and NO-dependent induction of penile erection is fully dependent on sGC. By comparing the results from this study with results obtained from a previous study using sGCa₁^-/- mice (were a remaining response could be observed to both exogenous and endogenous NO) we provide strong evidence for the contribution of the less abundantly expressed sGCa₂b₁ isoform in the mechanism of vasodilatation and penile erection. The unaltered responses to sGC-independent agents confirm the specificity of the impaired sGC-related responses observed in sGCb₁^ki/ki mice.