The effect of reactive oxygen species on contractions in beta-escin permeabilized rat detrusor was investigated. Cumulative calcium contractions were inhibited by hydrogen peroxide (H2O2) and hydroxyl (OH) but not by superoxide (O2-*). The sarcoplasmic reticulum calcium-ATPase inhibitor cyclopiazonic acid (CPA) and the mitochondrial blocker carbonyl cyanide p-trifluromethoxyphenylhydrazone (FCCP) decreased the calcium contractions, however in their presence, H2O2 and OH did not have further effect. Carbachol contractions were inhibited by either H2O2/OH/O-2 or CPA/FCCP. In the presence of CPA, carbachol contractions were not affected by H2O2 and OH but further decreased by O2Co. On the other hand, only H2O2 and OH elicited additional inhibition in carbachol responses in the presence of FCCP. Inositol triphosphate contraction was inhibited by OH whereas none of the radicals affect carbachol induced calcium sensitization. These results show that H2O2 and OH affects sarcoplasmic reticulum where O-2 acts on mitochondria to change contractions in rat detrusor smooth muscle.