The effects of racemic terodiline on isolated detrusor preparations from rabbit and man were compared with those of its (+)- and (-)-isomers, and with those of its main metabolite, parahydroxy-terodiline. Concentration-response (c-r) relations for carbachol and frequency-response relations for electrical stimulation were determined before and after addition of drugs. In preparations from both rabbit and man, all the drugs tested concentration-dependently shifted the c-r curve for carbachol to the right. (+)-Terodiline was more potent than (+/-)-terodiline, whereas (-)-terodiline and parahydroxy-terodiline were less potent. All drugs in concentrations greater than 10(-6) M had a non-competitive effect, depressing the maximum of the carbachol contraction. All drugs had a depressant effect on electrically evoked contractions. (+)-Terodiline was as effective (rabbit) or more effective (man) than (+/-)-terodiline, whereas (-)-terodiline and parahydroxy-terodiline were less effective. It is concluded that (+)-terodiline contributes to a main part of the detrusor effects of the racemate, and that part of this action is anticholinergic. Parahydroxy-terodiline had a profile of action similar to that of (+/-) terodiline, but its potency was low. Since it is present in plasma in low concentrations, its contribution to the clinical effects of terodiline is probably small. (+)-Terodiline may have a therapeutic potential.