Spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats were maintained on tap water or 1.5% NaCl for 3 weeks. During the high sodium regime 24-h urinary sodium excretion increased 10-fold and the basal blood pressure increased in the SHR. After 3 weeks the rats received arterial (carotid artery), venous and bladder catheters (suprapubic). Saline was infused continuously and in conscious rats atrial natriuretic peptide (alpha-hANP) was administered as bolus injections (8 and 16 nmol kg-1) and the blood pressure and heart rate and the urinary excretions of sodium, potassium (flame photometry), noradrenaline and dopamine (HPLC) were followed at 5-min intervals. The administration of ANP caused a short-lasting blood pressure reduction, tachycardia, diuresis and increased urinary excretions of sodium, potassium, noradrenaline and dopamine. The blood pressure responses to ANP did not differ between the rat strains, irrespective of the diet. The natriuresis and diuresis to ANP was reduced in animals on a high sodium diet, especially in the SHR. This may be interpreted as a down-regulation of target organ responsiveness to ANP during a high sodium diet and the inappropriately large decrease in the responsiveness that was observed in the SHR may be related to increase in blood pressure during the high sodium diet.