Recently, we showed that caveolin-1 (cav1) knockout (Cav1^-/-) mice have an impaired nitric oxide (NO) function in the longitudinal muscle (LM) layer of the small intestine. The defect was a reduced responsiveness of the muscles to NO, compensated by an increase in the function of apamin-sensitive, non-adrenergic, non-cholinergic (NANC) mediators. In the present study, we examined similarly the effects of cav1 knockout on the relaxation in circular muscle (CM) of the mouse small intestine. CM of Cav1^-/- mice also showed defective NO function, but less than in LM, as well as more activation of the apamin-sensitive NANC mediators. CM of Cav1^-/-, like LM, lacked cav1 but retained small amounts of caveolin-3 and caveolae in the outer CM layer. In addition we also examined the effects of a soluble guanylate cyclase (sGC) inhibitor, 1H-(1,2,4)oxadiazolo(4,3- A) quinazolin-1-one (ODQ), on the electric field stimulation (EFS)-mediated relaxation in both LM and CM. ODQ had an effect similar to the block of NO synthesis. Moreover, we compared the actions of two NO donors in the LM and CM of control and Cav1^-/- mice. Similar to LM, CM of Cav1^-/- showed a reduced responsiveness to the NO donors, sodium nitroprusside and S-nitroso-N-acetyl penicillamine. However, both ODQ and apamin blocked the inhibitory effects of the NO donors in LM, whereas apamin had no effect in CM. In conclusion, cav1 knockout affects NO function in both LM and CM but its effects in CM differ significantly.