DMPP inhibits the nerve-mediated contractions of the rat isolated bladder, its effect being greater in preparations from newborn (2 day old) than adult animals. This effect of DMPP was unaffected by hexamethonium. In preparations from adult animals the effect of DMPP increased with frequency of stimulation and was fully prevented by the presence of atropine. In bladders from newborn rats low concentrations of furthrethonium (FHR) (10 nM) activated a series of rhythmic contractions which were unaffected by tetrodotoxin and abolished by DMPP through an hexamethonium-insensitive action. On the other hand DMPP did not affect rhythmic contractions produced by a low concentration of eledoisin (60 nM). In bladders from adult rats FHR (10 microM) and KCI (30 mM) produced contractures of comparable magnitude. DMPP inhibited, in concentration-related manner the FHR-induced tonic contraction but had little effect on that produced by KCI. These findings indicate that in the rat bladder, DMPP antagonizes selectivity cholinergically-mediated contractions through a mechanism which is unaffected by hexamethonium or tetrodotoxin. An "atropine-like' activity of DMPP should be considered.