In isolated bladder smooth muscle from both rabbit and man, carbachol-induced contractions were reduced by the calcium channel blocker nifedipine, whereas the calcium channel promotor Bay K 8644 had no effect. In nominally calcium-free medium containing 10(-4) M EGTA, carbachol-induced contractions were reduced by 69% (rabbit) and 87% (man). These contractions were abolished by nifedipine, whereas Bay K 8644 significantly increased their amplitude, in rabbit preparations almost to control level. Electrical field stimulation produced contractions which could be suppressed by scopolamine by about 50% (rabbit) and more than 90% (man). These contractions were abolished by calcium-free medium (10(-4) M EGTA), suppressed by nifedipine, but significantly enhanced by Bay K 8644. The depressant effects of nifedipine, verapamil and diltiazem were reversed by Bay K 8644. The calcium channel blockers relaxed K+-induced contractions to base line, and this action was counteracted by Bay K 8644, less effectively when relaxations were induced by diltiazem. It is concluded that contractions produced by muscarinic receptor stimulation are primarily dependent on calcium bound to the outside of the membrane of the smooth muscle, and/or coming from the extracellular medium. Electrically evoked, scopolamine sensitive contractions seem to be mediated by a mechanism different from that of contractions produced by exogenously added muscarinic receptor agonist. The present data support the view that combined blockade of muscarinic receptors and calcium channels is an effective way of inhibiting bladder contractions in both rabbit and man.