In pentobarbital-anaesthetized rats (60 mg/kg, i.p.) renal pelvis distension with a pressure of 80 cm H2O caused a decline in mean arterial blood pressure. This pressure response, which disappeared rapidly after cessation of the distension, was used to study the effects of analgesic drugs known to be effective in renal colic pain in man. Morphine (0.75 and 1 mg/kg, s.c.) and the decapeptide caerulein (1.6, 4 and 8 microgram/kg, s.c.) abolished the pressure response. The effects of the largest doses lasted for at least 30 min. Ineffective in this respect were (a) desulphated caerulein (40 microgram/kg, s.c.) and (b) additional doses of pentobarbital (20 and 40 mg/kg, s.c.). This shows (a) the importance of the sulphated tyrosine (known from previous studies on central effects) and (b) the missing influence of the depth of anaesthesia. Naloxone (0.5 mg/kg, s.c.) abolished the effect of morphine (1 mg/kg, s.c.) but failed to influence that of caerulein (8 microgram/kg, s.c.). Even a fourfold dose of naloxone (2 mg/kg, s.c.) did not weaken the effect of caerulein. Naloxone, per se, was ineffective. These results suggest different mechanisms of the present effects of morphine and caerulein. It appears that renal pelvis distension in the anaesthetized rat can serve as a model of renal colic.