In this study, we have examined the effects of metamizol (dipyrone), a non-opioid analgesic which is effective in relieving renal colic pain, on nociceptive responses evoked by stimulation of the ureter, on pyeloureteral motility and on intraureter pressure after ureter obstruction in anaesthetised rats. Metamizol (5- 50 mg/kg i.v.) dose-dependently inhibited reflex pressor responses evoked by distensions of the ureter to pressures of 30, 55 and 75 mmHg for 30 s (ID50 = 8 +/- 1 mg/kg). Metamizol also dose-dependently reduced intraureter pressure during total ureter occlusion (25 mg/kg produced a reduction of 25% in 10 min). However, metamizol at doses up to 50 mg/kg had no effect on pyleoureteric motility (contraction amplitude, rate or intraureter pressure) under normal pressure conditions. We conclude that metamizol has a direct antinociceptive action on pain of ureteric origin, and spasmolytic effects after ureter obstruction (but not under normal conditions) which may also contribute to pain relief.