The presence and functional significance (if any) of spontaneous activity in the normal urinary bladder during filling is a controversial subject. One model used by many investigators to study spontaneous activity has been isolated urinary bladder smooth muscle strips. Although spontaneous activity is a property commonly observed in isolated urinary bladder strip preparations, the in vitro whole bladder preparation (rabbit) is devoid of spontaneous activity. Additionally, under normal conditions the in vivo rabbit bladder does not display spontaneous activity during the filling phases of micturition. The present study compares the spontaneous activity of isolated smooth muscle strips, the whole bladder preparation, and the catheterized in vivo bladder (rabbit). The results are as follows: 1) The spontaneous activity (frequency and amplitude) of isolated strips is extremely variable among strips of the same bladder. 2) Spontaneous activity is not affected by the following specific inhibitory compounds: tetrodotoxin, atropine, phentolamine, propranolol and hexamethonium. This indicates that spontaneous activity observed in isolated strips is myogenic in nature and not dependent on the activation of specific autonomic receptors. 3) The in vitro whole bladder preparation shows no spontaneous activity at any volume or pressure unless longitudinal tension is applied. 4) The spontaneous activity of the whole bladder subjected to longitudinal tension is not affected by the same compounds mentioned above. 5) Spontaneous activity of the in vivo bladder is absent at low intravesical volumes and pressures. Spontaneous activity develops upon reaching a critical pressure. However, this activity is completely inhibited by intravenous ganglionic blockade (hexamethonium). In the presence of hexamethonium, the in vivo bladder is devoid of spontaneous activity at any volume or pressure, thus the in vivo 'spontaneous activity' is mediated through neuronal reflexes. It is concluded that under normal circumstances the rabbit bladder is devoid of myogenic spontaneous activity and that the spontaneous activity observed in isolated strips is directly related to longitudinal stretch. Since under normal conditions the bladder is not subjected to longitudinal stretch, the spontaneous activity observed in the isolated strip studies has little physiological significance under normal conditions, but could help explain the pathophysiology of certain dysfunctions during the filling stage of micturition.