Dietary soy intake in man is proposed to provide cardiovascular protection, but it is not established whether this property is attributable to the soy protein per se or to associated dietary isoflavones. This investigation aimed to establish whether the dietary isoflavones in soy protein affect cardiovascular function. Ten days prior to mating, male and female Wistar rats were habituated to either a soy based isoflavone rich diet (plasma concentration 1.87 micromol l(-1) isoflavones) or the same diet after isoflavone elution (plasma isoflavone not detectable). Offspring were weaned onto and maintained on the same diet as their dam and sire for 6 months. Blood pressure, and constrictor and dilator responses in the aorta and mesenteric resistance arteries were assessed at 3 and 6 months of age. There was no effect of isoflavone removal from the diet on blood pressure, heart rate, aortic function or mesenteric artery contractile function, at either 3 or 6 months of age. Resistance mesenteric arteries from 6-month-old female rats fed the isoflavone rich diet demonstrated a modest increase in arterial distensibility compared with those fed the depleted diet, and mesenteric arteries from male and female rats fed the isoflavone rich diet showed increased sensitivity to acetylcholine. In summary, the isoflavone content of soy protein has no influence on blood pressure in healthy rats fed a diet based on soy protein, but influences small artery function.