Prostanoids, produced endogenously via cyclooxygenases, have been implicated in the sustained contraction of different smooth muscles. The two major types of cyclooxygenases are COX-1 and COX-2. The COX subtype involved in the basal state of the internal anal sphincter (IAS) smooth muscle tone is not known. To identify the COX subtype, we examined the effect of COX-1 vs. COX-2 selective inhibitors, SC-560 and rofecoxib respectively, on the rat IAS basal tone. We also determined the effect of selective deletion of COX-1 and COX-2 genes on the murine IAS basal tone. Data show that SC-560 causes significantly more efficacious and potent concentration-dependent decreases in the IAS tone as compared with rofecoxib. Supporting these data, significantly higher levels of COX-1 mRNA were found in the IAS vs. COX-2 mRNA. In addition, higher levels of COX-1 mRNA and protein were expressed in rat IAS vs. the rectal smooth muscle. In the wild type mice SC-560 (1x10^-5 M) caused 41.4 ± 3.4% (mean ± SE) decrease in the IAS tone vs. 5.4 ± 2.2% decrease caused by rofecoxib (p < 0.05; n = 5). The IAS basal tone from wild type mice was 0.172 ± 0.021 mN//mg, that decreased significantly to 0.080 ± 0.015 mN/mg in the COX-1^-/- mice (p < 0.05; n =5). However, the basal tone in the COX-2^-/- mice was not significantly different from their counterpart wild type. We conclude that COX-1-related products provide a significant contribution to the IAS tone.