To investigate the interaction between cardiovascular (CV)-reactive areas in the brain stem and urinary bladder (UB) motility, 48 adult cats of either sex were anesthetized intraperitoneally with alpha-chloralose (40 mg/kg) and urethan (400 mg/kg). The changes of UB motility and systemic arterial blood pressure (SAP) were produced by microinjection of sodium glutamate (0.5 M, 100-200 nl) into the pressor, depressor, or vagobradycardiac areas of the brain stem. Stimulation of these CV-reactive areas increased or decreased UB motility. Areas that produced an increase in UB motility listed in decreasing order of effectiveness are locus ceruleus-parabrachial nucleus in the pons, dorsal medulla, dorsal motor nucleus of vagus, and ventrolateral medulla. Areas eliciting a decrease in UB motility listed in decreasing order are gigantocellular tegmental field, parvocellular reticular nucleus, and ambiguus nucleus. Stimulation of other pressor sites in medulla also increased UB motility. Activation of the paramedian reticular nucleus, which consistently produced depressor responses, and activation of raphe nuclei, which produced depressor or pressor responses, consistently decreased UB motility. The integrity of the vagus nerve was not essential for the UB response to brain stimulation. These findings indicate that neuronal mechanisms for controlling UB and CV functions coexist at many sites in the brain stem. At those sites that commonly produce changes in UB motility, the type of UB response (excitation or inhibition) was in the same direction as the change of SAP. However, at some sites responses were inverse. It is not known whether the responses of the UB and CV system are controlled by common or separate populations of neurons at these sites.