The inhibitory effects of some muscarinic agonists on tritiated acetylcholine release evoked by field stimulation were investigated in the rat urinary bladder strip. The acetylcholine stores of the preparation were labelled with 3H-choline. Electrical field stimulation caused an outflow of tritium, reflecting the release of 3H-acetylcholine. The release of 3H-acetylcholine was decreased in a concentration-dependent manner by all the agonists tested: oxotremorine, muscarone, muscarine, carbachol and methylfurtrethonium. On the contrary, only muscarine and muscarone enhanced the basal efflux of tritium in a concentration-dependent fashion. Concentration-response curves were determined both at 2 Hz and at 1 Hz by using intermittent administration of the drugs. Maximal depression in release (by 78-82%) was observed in experiments at 1 Hz. A similar inhibition was obtained at 2 Hz frequency only when a low concentration of calcium (0.6 mM) in the medium was used. Oxotremorine was the most potent among the tested compounds with the same intrinsic activity as the other drugs. In contrast to the other agonists investigated, oxotremorine showed in about 10-fold greater potency at pre- than at postjunctional muscarine receptors in the rat urinary bladder. This difference might depend either on heterogeneity of muscarine receptors or on different mechanism(s) relating to the transducing properties of receptors at the pre- and postjunctional level. A comparison between the relative prejunctional potencies in the rat urinary bladder and in the guinea pig myenteric plexus (data from the literature) suggests that prejunctional muscarine receptors are similar in these tissues. Furthermore, the findings obtained with a low concentration of calcium in the medium may support the view that intraneuronal availability of calcium plays a significant role in modulating the prejunctional negative feed-back mechanism in the rat urinary bladder.