1. The influence of acute changes in salt balance on the cardiovascular effects of the calcium antagonist isradipine were investigated in two groups of 6 rabbits pretreated 24 hr before an acute experiment, one by substituting 0.45% NaCl for drinking water, the other one with a subcutaneous injection of 20 mg kg-1 furosemide. 2. On the next day the animals were anaesthetized with barbiturates and 10 ml kg-1 0.9% NaCl or 5% glucose were infused into the salt loaded and salt depleted animals respectively. 3. Regional blood flows were then measured with microspheres before and after i.v. administration of 3 and 10 micrograms kg-1 isradipine (PN200-110). 4. Salt depleted animals had a lower cardiac output and lower flows to several regional vascular beds including the kidneys. 5. The isradipine effects on systemic haemodynamic variables were not different in the two groups. 6. However, the effects on regional blood flows were modified. In salt loaded animals skeletal muscle flow increased more, starting from a higher baseline, yet flow to the kidneys, pancrease and spleen was higher before and decreased significantly more after the calcium antagonist. 7. The results suggest that an alteration in the sodium balance selectively affects the mechanism responsible for vascular tone in a few vascular beds.