1. Recent evidence has suggested that the impairment of endothelium-dependent cholinergic relaxation in vitro, which is seen in atherosclerotic large arteries of animals and man, could be part of a general deleterious effect to the endothelium of hypercholesterolaemia. 2. This possibility has been investigated in vitro by measuring the response to acetylcholine, sodium nitroprusside, 5-hydroxytryptamine and noradrenaline in segments of aorta and of femoral, mesenteric and cerebral small arteries (internal diameter approximately 200 microns) from control rabbits (n = 12) and from rabbits fed a 1% (w/w) cholesterol and 3% (w/w) coconut oil diet (n = 12) for 12 weeks. 3. Thoracic aorta segments from the control rabbits exhibited a maximal relaxation in response to acetylcholine of 64 +/- 11% compared with 10 +/- 5% (P less than 0.01) for thoracic segments from cholesterol-fed animals. Cerebral, femoral and mesenteric small arteries exposed to acetylcholine (10(-9)-10(-4) mol/l) relaxed to the same degree as arteries from control rabbits. The responses to sodium nitroprusside and bradykinin of the small arteries from the cholesterol-fed rabbits remained unaffected. 5-Hydroxytryptamine evoked comparable contractions in the small arteries, while the sensitivity to noradrenaline of the femoral small arteries was significantly decreased and the response of cerebral small arteries to noradrenaline in cholesterol-fed rabbits slightly increased compared with control rabbits. 4. Aorta from the cholesterol-fed rabbits had extensive atheromatous lesions. Morphological measurements and histological examination showed unchanged thickness and light microscopic appearance of intima and media of small arteries from cholesterol-fed animals compared with control animals. 5. The present study indicates that hypercholesterolaemia in this rabbit model is followed by atherosclerotic lesions and changed function of large arteries, but that both function and structure of systemic small arteries are largely unaffected.