Isolated renal arteries from newborn and adult guinea-pigs failed to respond to electrical stimulation of the intramural nerves, using parameters identical to those which elicit large neurogenic contractions in renal arteries from newborn and adult rabbits. The threshold stimulation frequency was lower and responses were greater in adult compared with neonatal rabbits. Comparisons of noradrenaline EC50 values showed that guinea-pig renal arteries are significantly less sensitive to noradrenaline that rabbit vessels. Quantitative fluorescence histochemistry of the perivascular adrenergic nerves in renal arteries revealed that the density of innervation gradually declines in the guinea-pig but substantially increases in the rabbit during postnatal development. The lack of neurogenic response of the guinea-pig renal artery, despite the existence of a varicose, adrenergic nerve plexus, is discussed.