The aim of the present study was to investigate whether or not activation of imidazoline receptors modulates noradrenaline release in the rat isolated kidney. Kidneys were pre-exposed to 3H-noradrenaline and the renal nerves were stimulated with 6 pulses at 100 Hz. The stimulation induced (S-I) outflow of radioactivity was taken as an index of endogenous noradrenaline release. The imidazoline derivatives clonidine (1-1000 nmol/l) and moxonidine (10-1000 nmol/l) inhibited S-I outflow of radioactivity with an EC50 of 6.8 nmol/l and 62.5 nmol/l and a maximum of 88% and 97%, respectively. The concentration response curves for clonidine and moxonidine were shifted to the right by the selective alpha 2-adrenoceptor antagonist rauwolscine (0.1 mumol/l) in a parallel manner with identical pKB's of 8.52 and 8.46, respectively. Furthermore, the alpha-adrenoceptor agonist (-)-alpha-methylnoradrenaline (0.1-30 nmol/l), which has no affinity for imidazoline binding sites, inhibited S-I outflow of radioactivity with and EC50 of 2.4 nmol/l and a maximum of 94%. Rauwolscine (0.1 mumol/l) again shifted the concentration response curve for this alpha-adrenoceptor agonist to the right with a pKB of 8.40. Moreover, the selective alpha 2-adrenoceptor antagonist 2-[2-(2-methoxy-1,4- benzo-dioxanyl)]imidazoline HCl (RX821002, 0.01 mumol/l) shifted the concentration response curves for clonidine and moxonidine to the right with pKB's of 9.46 and 9.18, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)