We have characterised pressor responses to stimulation of the ureter in anaesthetised rats (n = 20) as a model of acute visceral pain. The left ureter was cannulated close to the bladder and graded stimuli applied (5-90 mmHg, 30 s). The threshold was approximately 25 mmHg. Suprathreshold pressures evoked responses proportional to the stimulus intensity, which were little altered when stimulation of the kidney was prevented by ligation of the ureteric-pelvic junction. The stimulus response curve was dose-dependently attenuated by morphine (1-3 mg kg-1 i.v.), in a naloxone reversible manner. The characteristics of the responses observed correlate well with pain sensation in man and with the properties of ureteric primary afferent neurones in animals.