To examine the usefulness of Mn(2+) contrasted magnetic resonance imaging (MRI) in hypothalamic imaging, images obtained using T(1)-weighted MRI were compared with Fos expression, which is known to increase after activation of voltage-dependent Ca(2+) channels. Intravenous infusion of MnCl(2) elicited a rapid increase in the T(1)-weighted MRI signal intensity in the vessels and ventricles, but not in the brain parenchyma, suggesting that Mn(2+) did not diffuse freely across the blood-brain barrier. When the blood-brain barrier was broken by right intracarotid arterial injection of 25% D-mannitol, an increased signal intensity was seen in the right brain. Intracarotid arterial injection of hypertonic NaCl elicited rapid and striking increases in signal intensity in the paraventricular hypothalamic nucleus, supraoptic nucleus, and preoptic area, which are thought to be involved in central osmotic regulation. These observations were consistent with the Fos expression results. These results indicate that Mn(2+) contrasted MRI is a useful technique for investigating the autonomic center in the hypothalamus.